The most common and most likely answer to this crossword clue is the 4 letter word TRUE
I've seen this crossword clue in The Times
T
R
U
E
More clues leading to the result TRUE
- Yorkshire cricketer dropping a player? That's correct
- Nice street below front on the right
- EU right to upset firm
- Genuine regret by Tory leader
- Accurate time shown on French street
- Correct time on Paris street
- Valid ceasefire, the fourth to be abandoned
- At last felt regret to be genuine
- Time to regret being honest
- Loyal Conservative shunning armistice
Buddy explains!

The straight part
We know that "I agree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i agree = true.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRUE mean?
TRUE noun- The state of being in alignment.
- Truth.
- A pledge or truce.
- To straighten.
- To make even, level, symmetrical, or accurate, align; adjust.
- (of a statement) Conforming to the actual state of reality or fact; factually correct.
- Conforming to a rule or pattern; exact; accurate.
- Of the state in Boolean logic that indicates an affirmative or positive result.
- Loyal, faithful.
- Genuine; legitimate.
- Used in the designation of group of species, or sometimes a single species, to indicate that it belongs to the clade its common name (which may be more broadly scoped in common speech) is restricted to in technical speech, or to distinguish it from a similar species, the latter of which may be called false.
- (of an aim or missile in archery, shooting, golf etc.) Accurate; following a path toward the target.
- Fair, unbiased, not loaded.
- (Of a literary genre) based on actual historical events.
- (of shooting, throwing etc) Accurately.