Cry about a girl's horse (7)
I believe the answer is:
cavalry
'horse' is the definition.
(I know that horse can be written as cavalry)
'cry about a girl's' is the wordplay.
'about' means one lot of letters goes inside another.
'girl' becomes 'val' (short for Valerie).
'a'+'val'='aval'
'cry' placed around 'aval' is 'CAVALRY'.
(Other definitions for cavalry that I've seen before include "Horse troops" , "Horse soldiers" , "Mounted troops" , "Mounted soldiers" , "Horse-soldiers" .)