Relieve area captured by friendly force (5)
I believe the answer is:
allay
'relieve' is the definition.
(synonyms)
'area captured by friendly force' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'captured by' indicates putting letters inside.
'friendly force' becomes 'ally' (I've seen this in another clue).
'a' going into 'ally' is 'ALLAY'.
(Other definitions for allay that I've seen before include "Alleviate (hunger)" , "Diminish or end (fears)" , "Calm, put at rest" , "Assuage, ease" , "Diminish (fears/concern)" .)