Straighten back of jacket? I would, certainly (4,2)
I believe the answer is:
tidy up
'straighten' is the definition.
('tidy up' can be a synonym of 'straighten')
'back of jacket? i would certainly' is the wordplay.
'back of' says to take the final letters.
'i would' becomes 'id'.
'certainly' becomes 'yup'.
The final letter of 'jacket' is 't'.
't'+'id'+'yup'='TIDY UP'
(Other definitions for tidy up that I've seen before include "Put things back in their places" , "Make neater" , "Make neat" , "Put (things or places) in order" , "Spring clean" .)