Grandpa, oddly fit, is a pleasure-seeker (8)
I believe the answer is:
gadabout
'a pleasure-seeker' is the definition.
(gadabout can mean a pleasure-seeker)
'grandpa oddly fit' is the wordplay.
'oddly' indicates alternate letters (letters in odd-numbered positions).
'fit' becomes 'bout' (I've seen this in other clues).
The alternate letters of 'grandpa' are 'gada'.
'gada'+'bout'='GADABOUT'
'is' acts as a link.
(Other definitions for gadabout that I've seen before include "Person who wanders aimlessly in search of pleasure" , "Restless seeker after amusement" , "Pleasure-seeker" , "restless type" , "Vagrant" .)