Make adjustments to the bottom of chainsaw when cutting wood (5)
I believe the answer is:
tweak
'make adjustments to' is the definition.
(to tweak something is to adjust it)
'the bottom of chainsaw when cutting wood' is the wordplay.
'the bottom of' indicates one should take the final letters.
'when cutting' indicates putting letters inside.
'wood' becomes 'teak' (teak is a kind of wood).
The last letter of 'chainsaw' is 'w'.
'w' inserted inside 'teak' is 'TWEAK'.
(Other definitions for tweak that I've seen before include "Make minor adjustments" , "Edit" , "Twist; adjust finely" , "Nip" , "Modify slightly" .)