Having fun in a boat going in to fish (7)
I believe the answer is:
larking
'having fun' is the definition.
(I've seen this before)
'boat going in to fish' is the wordplay.
'boat' becomes 'ark' (ark is a kind of boat).
'going in' means one lot of letters goes inside another.
'to fish' becomes 'ling' (ling is a kind of fish).
'ark' put within 'ling' is 'LARKING'.
'in a' is the link.
(Other definitions for larking that I've seen before include "hunting for birds" , "Frolicking like a bird" , "having fun" .)