Bag is right one for GP to possess (4)
I believe the answer is:
grip
'bag' is the definition.
(type of travelling bag)
'right one for gp to possess' is the wordplay.
'right' becomes 'r' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'for' says to put letters next to each other.
'to possess' means one lot of letters goes inside another.
'r'+'i'='ri'
'ri' placed into 'gp' is 'GRIP'.
'is' is the link.
(Other definitions for grip that I've seen before include "Purchase" , "Ditch" , "Take firm hold of travelling bag?" , "Baggage - hold" , "Fascinate" .)