The most common and most likely answer to this crossword clue is the 10 letter word EXPATIATED
I've seen this crossword clue in The Daily Telegraph
E
X
P
A
T
I
A
T
E
D
Buddy explains!
The straight part
We know that "Held forth on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case held forth on = expatiated.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 EXPATIATED mean?
EXPATIATED verb- To range at large, or without restraint.
- To write or speak at length; to be copious in argument or discussion.
- To expand; to spread; to extend; to diffuse; to broaden.