Fairly easily, more or less (6,4)
I believe the answer is:
pretty well
'more or less' is the definition.
(I've seen this before)
'fairly easily' is the wordplay.
'fairly' becomes 'pretty' (I've seen this before).
'easily' becomes 'well' (eg in a phrase like 'perfectly well').
'pretty'+'well'='PRETTY WELL'
(Other definitions for pretty well that I've seen before include "Close to" , "Sort of" , "More or less" , "Practically" , "Aim of dressing in Derbyshire" .)