Relative aboard sizable vessels? They drift around (8)
I believe the answer is:
vagrants
'they drift around' is the definition.
The definition suggests an adverb but the answer is not.
'relative aboard sizable vessels?' is the wordplay.
'relative' becomes 'gran' (short for grandmother).
'aboard' means one lot of letters goes inside another.
'sizable vessels?' becomes 'vats' (vat is a kind of vessel. I am not sure about the 'sizable' bit.).
'gran' placed inside 'vats' is 'VAGRANTS'.
Can you help me to learn more?
(Other definitions for vagrants that I've seen before include "Drifters, vagabonds" , "Drifters with no apparent means of support" , "Homeless types" , "Wanderers, tramps" .)