Old relief vessel certain to get publicity (8)
I believe the answer is:
exposure
'publicity' is the definition.
(I've seen this before)
'old relief vessel certain to get' is the wordplay.
'old' becomes 'o' (abbreviation).
'relief vessel' becomes 'exp' (I can't explain this - if you can you should give a lot more credence to this answer).
'certain' becomes 'sure' (synonyms).
'to get' means one lot of letters goes inside another.
'exp'+'sure'='expsure'
'o' placed within 'expsure' is 'EXPOSURE'.
Can you help me to learn more?
(Other definitions for exposure that I've seen before include "Act of laying open or bare" , "Uncovering" , "Vulnerability; photograph" , "Openness" , "Lack of shelter" .)