His involvement in walkabout on vacation is a game (5)
I believe the answer is:
whist
'game' is the definition.
(whist is a card game)
'his involvement in walkabout on vacation' is the wordplay.
'involvement in' means one lot of letters goes inside another (inserted letters get involved in others).
'on vacation' suggests removing the centre (after the inside letters have vacated the word).
'walkabout' with its centre taken out is 'wt'.
'his' inserted into 'wt' is 'WHIST'.
'is a' is the link.
(Other definitions for whist that I've seen before include "Card game for four" , "Tricky card game" , "calls for silence?" , "Demand silence" , "Trick-taking card game" .)