Look after old trains union (London Region) (5)
I believe the answer is:
nurse
'look after' is the definition.
(I've seen this before)
'old trains union london region' is the wordplay.
'old trains union' becomes 'NUR' (National Union of Railwaymen).
'london region' becomes 'SE' (South East postcode).
'nur'+'se'='NURSE'
(Other definitions for nurse that I've seen before include "Medical carer" , "Suckle" , "Hospital carer" , "Nourish" , "drink slowly" .)