Figure the horse to back is “Sea Bird” (6)
I believe the answer is:
gannet
'is sea bird' is the definition.
(I've seen this before)
'figure the horse to back' is the wordplay.
'figure' becomes 'ten' (I've seen this in other clues).
'the horse' becomes 'nag' (nag is a kind of horse).
'to back' is a reversal indicator.
'ten'+'nag'='tennag'
'tennag' reversed gives 'GANNET'.
(Other definitions for gannet that I've seen before include "Solan goose was this bird's former name" , "Heavily built marine bird" , "Act in accordance (with command)" , "Marine bird - greedy person" , "guzzler?" .)