Mend publicity about Royal Navy (4)
I believe the answer is:
darn
'mend' is the definition.
(darning is a kind of mending)
'publicity about royal navy' is the wordplay.
'publicity' becomes 'ad' (advert).
'about' shows that the letters should be reversed in order.
'royal navy' becomes 'rn'.
'ad' back-to-front is 'da'.
'da'+'rn'='DARN'
(Other definitions for darn that I've seen before include "Stitch" , "Mend (e.g. socks)" , "curse" , "Mend by stitching - damn!" , "Mend (woollens)" .)