Mostly loyal worker becomes absentee (6)
I believe the answer is:
truant
'absentee' is the definition.
(truant is a kind of absentee)
'mostly loyal worker' is the wordplay.
'mostly' means to remove the last letter.
'loyal' becomes 'true' (I've seen this before).
'worker' becomes 'ant' (worker is a type of ant**).
'true' with its final letter taken away is 'tru'.
'tru'+'ant'='TRUANT'
'becomes' is the link.
(Other definitions for truant that I've seen before include "Academic absentee" , "Pupil absent without permission" , "not learning lesson" , "Kid bunking off school" , "Inexcusably absent" .)