First month in group upset dependable people (7)
I believe the answer is:
trojans
'dependable people' is the definition.
(Trojan can mean a reliably hard-working person)
'first month in group upset' is the wordplay.
'first month' becomes 'jan'.
'in' indicates putting letters inside.
'group' becomes 'sort'.
'upset' says the letters should be written backwards.
'sort' backwards is 'tros'.
'jan' put inside 'tros' is 'TROJANS'.
(Other definitions for trojans that I've seen before include "Hard workers" , "Hard-working types" , "Citizens of ancient city in Asia Minor invaded by Wooden Horse" , "People of Paris" , "noted for hard work" .)