Back final Derby finisher, say, having squandered lead (7)
I believe the answer is:
endorse
'back' is the definition.
(synonyms)
'final derby finisher say having squandered lead' is the wordplay.
'final' becomes 'end' (synonyms).
'derby finisher say' becomes 'horse' (a horse might race at Derby racecourse).
'having squandered lead' means to remove the first letter (the letter which leads the word is removed).
'horse' with its initial letter removed is 'orse'.
'end'+'orse'='ENDORSE'
(Other definitions for endorse that I've seen before include "Add one's authority to" , "Sore end (anag.)" , "Affirm" , "Give public approval of (something)" , "Express approval" .)