Control temperature for weakling (4)
I believe the answer is:
'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).
(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" .)