Try to catch a fish, small fish with no head or tail (6)
I believe the answer is:
tickle
'try to catch a fish' is the definition.
(as in trout tickling)
'small fish with no head or tail' is the wordplay.
'small' becomes 'tick' (I can't explain this - if you can you should believe this answer much more).
'fish' becomes 'sticklebacks'.
'with no head or tail' indicates the central letters (the head and tail of the word are removed).
The middle letters of 'sticklebacks' are 'le'.
'tick'+'le'='TICKLE'
(Other definitions for tickle that I've seen before include "Excite by touch" , "Cause giggling by touching lightly" , "Stroke lightly to make laugh" , "Excite pleasurably" , "Catch (trout) by rubbing" .)