Small party seen in wood only occasionally (6)
I believe the answer is:
seldom
'only occasionally' is the definition.
(I've seen this before)
'small party seen in wood' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'party' becomes 'do' (do is a kind of party).
'seen in' indicates putting letters inside.
'wood' becomes 'elm' (elm is a kind of wood).
'do' put inside 'elm' is 'eldom'.
's'+'eldom'='SELDOM'
(Other definitions for seldom that I've seen before include "Not very often, rarely" , "on rare occasions" , "Infrequent" , "Not 21" , "Once in a blue moon" .)