Time's up - that's below the belt! (5)
I believe the answer is:
thigh
'that's below the belt' is the definition.
'time's up' is the wordplay.
'time' becomes 't'.
'up' becomes 'high' ('high' is associated in meaning with 'up').
't'+'high'='THIGH'
(Other definitions for thigh that I've seen before include "Joint of meat" , "part of body" , "Part of member" , "Leg between knee and hip" , "Upper joint of the leg" .)