Unruly form tucked into favourite fish (7)
I believe the answer is:
pomfret
'fish' is the definition.
(pomfret is a kind of fish)
'unruly form tucked into favourite' is the wordplay.
'unruly' indicates anagramming the letters.
'tucked into' is an insertion indicator.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'form' is an anagram of 'omfr'.
'omfr' inserted within 'pet' is 'POMFRET'.
(Other definitions for pomfret that I've seen before include "Round flat liquorice sweet - Asian food fish" , "sort of cake" , "cake, Yorkshire style" .)