Yell when a girl is trapped — troops to the rescue? (7)
I believe the answer is:
cavalry
'troops to the rescue?' is the definition.
(send in the cavalry)
'yell when a girl is trapped' is the wordplay.
'yell when' becomes 'cary' (I can't explain this - if you can you should believe this answer much more).
'a girl' becomes 'Val' (short for Valerie).
'is trapped' indicates putting letters inside.
'cary' placed around 'val' is 'CAVALRY'.
(Other definitions for cavalry that I've seen before include "Military unit" , "Mounted soldiers" , "Horse troops" , "Horse soldiers" , "Mounted troops" .)