Fish’s barnacle requires regular trimming perhaps (9)
I believe the answer is:
perchance
'perhaps' is the definition.
(synonyms)
'fish's barnacle' is the wordplay.
'fish' becomes 'perch' (perch is a kind of fish).
'barnacle' becomes 'ance' (I can't explain this - if you can you should believe this answer much more).
'perch'+'ance'='PERCHANCE'
'requires regular trimming' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for perchance that I've seen before include "As it may happen" , "'To sleep: -------- to dream:' Hamlet" , "Maybe (to dream? - Hamlet)" , "possibly" .)