Large union leader with doctor in the past getting back pain (7)
I believe the answer is:
lumbago
'back pain' is the definition.
(I've seen this in another clue)
'large union leader with doctor in the past' is the wordplay.
'leader' indicates taking the first letters.
'with' means one lot of letters go next to another.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in the past' becomes 'ago' (three years ago means three years in the past).
The first letters of 'large union' is 'lu'.
'lu'+'mb'+'ago'='LUMBAGO'
'getting' is the link.
(Other definitions for lumbago that I've seen before include "Painful condition" , "Maul bog badly for a rheumatic affliction" , "Back complaint" , "Back problem" , "Pain -- a bum log (anag)" .)