Tons lacking power to make changes (5)
I believe the answer is:
tweak
'make changes' is the definition.
'tons lacking power' is the wordplay.
'tons' becomes 't'.
'lacking power' becomes 'weak' ().
't'+'weak'='TWEAK'
'to' acts as a link.
(Other definitions for tweak that I've seen before include "Make fine adjustment" , "Fine-tune, twist" , "Pinch sharply" , "Edit" , "spin, thus?" .)