Top horse I backed, Jack perhaps on it (8)
I believe the answer is:
cardigan
'top' is the definition.
(I've seen this before)
'horse i backed jack perhaps on it' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse**).
'backed' is a reversal indicator.
'jack perhaps' becomes 'card' (jack is a playing card).
'on it' means one lot of letters go next to another.
'nag'+'i'='nagi'
'nagi' reversed gives 'igan'.
'igan' put after 'card' is 'CARDIGAN'.
(Other definitions for cardigan that I've seen before include "Woollen jacket" , "Arcading in woolly garment" , "location of Welsh bay" , "Knitted garment" , "See 25" .)