Lack of volunteers to join railway -- official (6)
I believe the answer is:
notary
'official' is the definition.
(notary is a kind of official)
'lack of volunteers to join railway' is the wordplay.
'lack of' becomes 'no' (lack of money is no money).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'to join' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'no'+'ta'+'ry'='NOTARY'
(Other definitions for notary that I've seen before include "One drawing up legal documents" , "Official working on legal documents" , "Legal functionary" , "Official authorised to witness signing of documents" , "legal worker" .)