Back Cockney's filly after tip (7)
I believe the answer is:
endorse
'back' is the definition.
(back can mean to support or endorse)
'cockney's filly after tip' is the wordplay.
I cannot quite see how this works, but
'tip' could be 'end' (tip can mean the end of something) and 'end' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for endorse that I've seen before include "Ratify" , "Add one's authority to" , "Countersign" , "Express approval" , "No reeds (anag)" .)