Slightly adjust end of saw penetrating wood (5)
I believe the answer is:
tweak
'slightly adjust' is the definition.
(I've seen this before)
'end of saw penetrating wood' is the wordplay.
'end of' indicates one should take the final letters.
'penetrating' is an insertion indicator.
'wood' becomes 'teak' (teak is a kind of wood).
The last letter of 'saw' is 'w'.
'w' placed inside 'teak' is 'TWEAK'.
(Other definitions for tweak that I've seen before include "Pull and twist" , "Fine tune" , "Make small adjustments" , "Fine adjustment (colloq.)" , "Pinch sharply" .)