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