Trendy fish found underfoot? (6)
I believe the answer is:
insole
'found underfoot?' is the definition.
(insoles are worn under one's feet)
'trendy fish' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'fish' becomes 'sole' (sole is a kind of fish).
'in'+'sole'='INSOLE'
(Other definitions for insole that I've seen before include "Boots may be holding it" , "Oxford's content" , "Padding in a shoe" , "Lesion in part of foot" , "found in boot" .)