Rarely perform in small wood (6)
I believe the answer is:
seldom
'rarely' is the definition.
(I know that rarely can be written as seldom)
'perform in small wood' is the wordplay.
'perform' becomes 'do' (both can mean complete an action).
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'wood' becomes 'elm' (elm is a type of tree).
's'+'elm'='selm'
'do' going inside 'selm' is 'SELDOM'.
(Other definitions for seldom that I've seen before include "Not 21" , "Alluvium" , "Models (anag)" , "Not often, infrequently" , "Not very often, rarely" .)