Offer to join infantry as rookie (10)
I believe the answer is:
tenderfoot
'rookie' is the definition.
(thesaurus)
'offer to join infantry' is the wordplay.
'offer' becomes 'tender' (tender offer is a kind of offer).
'to join infantry' becomes 'foot' (I've seen this in other clues).
'tender'+'foot'='TENDERFOOT'
'as' is the link.
(Other definitions for tenderfoot that I've seen before include "one still finding her bearings" , "Rookie" , "Newcomer, novice" , "Novice (chiefly US)" , "new employee of mine" .)