Just small pianos (8)
I believe the answer is:
uprights
'pianos' is the definition.
(upright is a kind of piano)
'just small' is the wordplay.
'just' becomes 'upright' ('upright' can be a synonym of 'just').
'small' becomes 's' (abbreviation - e.g. clothes size).
'upright'+'s'='UPRIGHTS'
(Other definitions for uprights that I've seen before include "Vertical posts as in rugby goal" , "Vertical posts as for rugby match" , "Vertical posts or stakes" , "goalposts?" , "Vertical posts in rugby pitch" .)