Manipulate thickness (3)
I believe the answer is:
ply
This clue is a double definition.
'manipulate' is the first definition.
(I know that ply is a kind of handle)
'thickness' is the second definition.
(I have seen 'Thickness of yarn ' mean 'ply' so perhaps 'thickness' could also mean 'ply')
(Other definitions for ply that I've seen before include "'Persist in offering, liquor say (3)'" , "carry on" , "Laminate thickness" , "Layer of folded material" , "Strand, of wool or wood, say" .)