Rookie in bid to join infantry (10)
I believe the answer is:
tenderfoot
'rookie' is the definition.
(thesaurus)
'bid to join infantry' is the wordplay.
'bid' becomes 'tender' ('tender' can be a synonym of 'bid').
'to join infantry' becomes 'foot' (I've seen this in other clues).
'tender'+'foot'='TENDERFOOT'
'in' acts as a link.
(Other definitions for tenderfoot that I've seen before include "Raw recruit" , "Greenhorn" , "Newcomer, novice" , "one still finding her bearings" , "Novice (chiefly US)" .)