She'll back us for a close finish (3)
I believe the answer is:
sue
'she'll' is the definition.
I can't tell whether this defines the answer.
'back us for a close finish' is the wordplay.
'back' is a reversal indicator.
'for' is a charade indicator (letters next to each other).
'finish' indicates one should take the final letters.
The last letter of 'close' is 'e'.
'us' written backwards gives 'su'.
'su'+'e'='SUE'
Can you help me to learn more?
(Other definitions for sue that I've seen before include "Take court action" , "Little Susan" , "Appeal" , "Prosecute at law" , "Take legal case against" .)