Clue: Post returned by British sailors, ashore at last for realistic treasury man (4,5)
Answer: LIAM BYRNE
Explanation: Post returned by British sailors, ashore at last for realistic treasury man (4,5) 
LIAM (MAIL returned) BY RN (British sailors – Royal Navy) (ashor)E. (Liam Byrne was Chief Secretary to the Treasury under Gordon Brown – *not sure why "realistic" is there*?)