Reversing right round a sailor (3)
I believe the answer is:
tar
'sailor' is the definition.
(tar is a kind of sailor)
'reversing right round a' is the wordplay.
'reversing' says the letters should be written backwards.
'right' becomes 'rt'.
'round' means one lot of letters goes inside another.
'rt' enclosing 'a' is 'rat'.
'rat' written backwards gives 'TAR'.
(Other definitions for tar that I've seen before include "Road surfacing or old sailor" , "Old salt, sailor" , "Seaman; pitch" , "Sticky road material or informal sailor" , "'Thick, dark, sticky liquid (3)'" .)