Temperature not hard to adjust (5)
I believe the answer is:
tweak
'adjust' is the definition.
(I've seen this before)
'temperature not hard' is the wordplay.
'temperature' becomes 't' (abbreviation).
'not hard' becomes 'weak' (weak is the opposite of hard).
't'+'weak'='TWEAK'
'to' is the link.
(Other definitions for tweak that I've seen before include "Make small adjustments" , "Make minor adjustment" , "Fine tune" , "Fine-tune, twist" , "[TUG]" .)