Mark tucks into a scotch — that's better (5)
I believe the answer is:
amend
'better' is the definition.
('amend' can be a synonym of 'better')
'mark tucks into a scotch' is the wordplay.
'mark' becomes 'm'.
'tucks into' is an insertion indicator.
'scotch' becomes 'end' ('scotch' can mean to put an end to something).
'a'+'end'='aend'
'm' inserted into 'aend' is 'AMEND'.
'that's' is the link.
(Other definitions for amend that I've seen before include "Make small improvements to" , "Alter in writing" , "Make minor changes to" , "Change, alter" , "Rectify" .)