Root posh sailors planted in dump (6)
I believe the answer is:
turnip
'root' is the definition.
(I know that turnip is a root vegetable)
'posh sailors planted in dump' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'planted in' indicates putting letters inside.
'dump' becomes 'tip' (as in a rubbish tip).
'u'+'rn'='urn'
'urn' inserted into 'tip' is 'TURNIP'.
(Other definitions for turnip that I've seen before include "Watch" , "Rip nut up for a vegetable" , "Plant with round white root used as vegetable" , "'Root vegetable, yellow or white (6)'" , "veggie" .)