Slightly change start of walk in wood (5)
I believe the answer is:
tweak
'slightly change' is the definition.
(to tweak something is to make a small change to it)
'start of walk in wood' is the wordplay.
'start of' says to take the initial letters.
'in' indicates putting letters inside.
'wood' becomes 'teak' (type of timber).
The first letter of 'walk' is 'w'.
'w' placed inside 'teak' is 'TWEAK'.
(Other definitions for tweak that I've seen before include "[TUG]" , "Make fine adjustment" , "Pinch sharply" , "Minor change" , "Slightly alter" .)