One wild to become prostrate (3,4)
I believe the answer is:
lie down
'prostrate' is the definition.
(thesaurus)
'one wild to become' is the wordplay.
'to become' indicates anagramming the letters.
'one'+'wild'='onewild'
'onewild' is an anagram of 'LIE-DOWN'.
(Other definitions for lie down that I've seen before include "Rest in a horizontal position" , "Enid Low (anag.)" , "Assume recumbent position" , "joining rest" , "Relax" .)