The most common and most likely answer to this crossword clue is the 2 plus 3 plus 5 letter word ON THE BRAIN
I've seen this crossword clue in The Times
O
N
T
H
E
B
R
A
I
N
Buddy explains!

The straight part
We know that "constantly in mind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case constantly in mind = on the brain.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.