Labour's first union doctor, in the past, was a pain (7)
I believe the answer is:
lumbago
'a pain' is the definition.
(I know that lumbago is a type of backache)
'labour's first union doctor in the past' is the wordplay.
'labour's first' becomes 'l' (1st letter of 'labour').
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'in the past' becomes 'ago' (synonyms).
'l'+'u'+'mb'+'ago'='LUMBAGO'
'was' is the link.
(Other definitions for lumbago that I've seen before include "Back problem" , "Back complaint" , "Rheumatic backache" , "Lower back ache" , "Maul bog badly for a rheumatic affliction" .)