Absentee? Mostly loyal worker (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 off is 'tru'.
'tru'+'ant'='TRUANT'
(Other definitions for truant that I've seen before include "One cutting school" , "Pupil absent without permission" , "Child who stays away from school without leave" , "Pupil staying away from school" , "School avoider" .)