Young lady repelling favourite put foot in it? (7)
I believe the answer is:
misstep
'put foot in it?' is the definition.
(both can mean to make a mistake)
'young lady repelling favourite' is the wordplay.
'young lady' becomes 'miss' (synonyms).
'repelling' is a reversal indicator.
'favourite' becomes 'pet' (synonyms).
'pet' reversed gives 'tep'.
'miss'+'tep'='MISSTEP'
(Other definitions for misstep that I've seen before include "False move" , "Badly judged footing" , "Badly judged move" , "Error" , "bad move" .)