Decrease power in adjustment of rate (5)
I believe the answer is:
taper
'decrease' is the definition.
(tapering is a kind of decreasing)
'power in adjustment of rate' is the wordplay.
'power' becomes 'p'.
'in' is an insertion indicator.
'adjustment of' indicates anagramming the letters (I've seen this in other clues).
'rate' anagrammed gives 'taer'.
'p' going into 'taer' is 'TAPER'.
(Other definitions for taper that I've seen before include "Become thinner towards one end" , "Thin progressively" , "Tail off; a spill" , "Narrow towards the end" , "Get more slender" .)