Nice girl, she'll help fellows out (6)
I believe the answer is:
maiden
'nice girl she'll' is the definition.
I can't tell whether this definition defines the answer.
'help fellows out' is the wordplay.
'help' becomes 'aid' (synonyms).
'fellows' becomes 'men' (fellow can mean a man).
'out' means one lot of letters goes inside another.
'aid' going into 'men' is 'MAIDEN'.
Can you help me to learn more?
(Other definitions for maiden that I've seen before include "Unmated; runless over" , "First ever public address" , "Earliest - unmarried" , "No runs are scored from this - young girl" , "To the rear (of a ship)" .)