Control temperature for weakling (4)
I believe the answer is:
runt
'for weakling' is the definition.
(I've seen this before)
'control temperature' is the wordplay.
'control' becomes 'run' (I've seen this before).
'temperature' becomes 't' (abbreviation).
'run'+'t'='RUNT'
(Other definitions for runt that I've seen before include "stunted figure" , "Littlest oinker" , "Turn into smallest of the litter" , "Weakest animal in a litter" , "little piggy" .)