American girl's returns fall behind (3)
I believe the answer is:
lag
'fall behind' is the definition.
(synonyms)
'american girl's returns' is the wordplay.
'american girl' becomes 'gal' (US term for a girl).
'returns' is a reversal indicator.
'gal' backwards is 'LAG'.
(Other definitions for lag that I've seen before include "Drop behind" , "Delay" , "trail" , "Go too slow, fail to keep up" , "Provide protection" .)