Fighting military first who are ardent (4)
I believe the answer is:
warm
'are ardent' is the definition.
(I've seen this before)
'fighting military first' is the wordplay.
'fighting' becomes 'war'.
'first' indicates taking the first letters.
The first letter of 'military' is 'm'.
'war'+'m'='WARM'
'who' acts as a link.
(Other definitions for warm that I've seen before include "easily aroused" , "Slightly hot" , "Getting close to target" , "Moderately hot" , "Neither hot nor cold" .)