Absolutely certain woman is to reject first soldier (7)
I believe the answer is:
adamant
'absolutely certain' is the definition.
'woman is to reject first soldier' is the wordplay.
'woman' becomes 'ada'.
'is to reject first' becomes 'm' (I can't explain this - if you can you should give a lot more credence to this answer).
'soldier' becomes 'ant' (soldier is a type of ant).
'ada'+'m'+'ant'='ADAMANT'
(Other definitions for adamant that I've seen before include "Refusing to budge" , "Inflexible: fabulously hard rock" , "popstar" , "unshakeable in purpose" , "uncompromising" .)