Mostly, then, I work to get a degree in (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(I've seen this before)
'then i work to get a degree in' is the wordplay.
'work' is an anagram indicator.
'to get' means one lot of letters go next to another.
'a degree' becomes 'ma' (Master of Arts is a type of degree**).
'then'+'i'='theni'
'theni' anagrammed gives 'inthe'.
'inthe'+'ma'+'in'='IN THE MAIN'
(Other definitions for in the main that I've seen before include "Where those who walked the plank finished" , "Where ships are" , "seaborne?" , "Swimming" , "Almost entirely" .)