Old trade union to celebrate caring profession (7)
I believe the answer is:
nursing
'caring profession' is the definition.
(I've seen this before)
'old trade union to celebrate' is the wordplay.
'old trade union' becomes 'nur' (National Union of Railwaymen).
'to celebrate' becomes 'sing' (I've seen this before).
'nur'+'sing'='NURSING'
(Other definitions for nursing that I've seen before include "Feeding baby" , "Patient care" , "now a graduate profession" , "Medical vocation" , "sister's career" .)