Line up, exhausted, after foxtrot (4,2)
I believe the answer is:
fall in
'line up' is the definition.
(thesaurus)
'exhausted after foxtrot' is the wordplay.
'exhausted' becomes 'all in' ('all in' can mean exhausted).
'after' says to put letters next to each other.
'foxtrot' becomes 'f' (phonetic alphabet: alpha, bravo, charlie etc.).
'allin' after 'f' is 'FALL-IN'.
(Other definitions for fall in that I've seen before include "Military order to join formation" , "get dressed?" , "Army order to take place in formation" , "Order to troops to take place in formation" , "'Take one's place in a military line (4,2)'" .)