Female soldier's extremely regimental (4)
I believe the answer is:
girl
'female' is the definition.
(girl is a kind of female)
'soldier's extremely regimental' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'extremely' says to hollow out the word (remove centre letters).
'regimental' with its middle taken out is 'rl'.
'gi'+'rl'='GIRL'
(Other definitions for girl that I've seen before include "Maid" , "Young woman" , "Maidservant" , "Female child" , "A young female" .)