It's beastly being a long time in one occupation (7)
I believe the answer is:
leonine
'it's beastly' is the definition.
(I've seen this before)
'a long time in one occupation' is the wordplay.
'a long time' becomes 'eon' (eon is a kind of long time).
'in' is an insertion indicator.
'one occupation' becomes 'line' (line can mean someone's occupation or industry).
'eon' going inside 'line' is 'LEONINE'.
'being' is the link.
(Other definitions for leonine that I've seen before include "Relating to a carnivore or to certain popes" , "Beastly" , "None lie (anag) - like a lion" , "Pope" , "As a maned animal" .)