Setter's contrarily breaking engagement to take another course (7)
I believe the answer is:
deviate
'take another course' is the definition.
(to deviate is to change course)
'setter's contrarily breaking engagement' is the wordplay.
'setter's' becomes 'ive' ('I've', referring to the crossword setter).
'contrarily' shows that the letters should be reversed in order (letters in the opposite or contrary direction).
'breaking' means one lot of letters goes inside another.
'engagement' becomes 'date' (synonyms).
'ive' reversed gives 'evi'.
'evi' inserted inside 'date' is 'DEVIATE'.
'to' is the link.
(Other definitions for deviate that I've seen before include "Turn away" , "Depart from an established course" , "Turn aside or digress from" , "Go off course" , "Differ" .)