After half a minute we become deficient (5)
I believe the answer is:
minus
'deficient' is the definition.
(thesaurus)
'after half a minute we' is the wordplay.
'after' is a charade indicator (letters next to each other).
'half a' means to take half the letters.
'we' becomes 'us'.
'minute' with half the letters taken is 'min'.
'min' next to 'us' is 'MINUS'.
'become' is the link.
(Other definitions for minus that I've seen before include "deficiency" , "Lacking; below zero" , "Negative sign - negative thing" , "Reduced by the subtraction of - thus?" , "Less, without" .)