Five help to get piano in flat (5)
I believe the answer is:
vapid
'flat' is the definition.
('vapid' can be a synonym of 'flat')
'five help to get piano' is the wordplay.
'five' becomes 'v' (roman numeral).
'help' becomes 'aid' ('aid' can be a synonym of 'help'**).
'to get' means one lot of letters goes inside another.
'piano' becomes 'p'.
'aid' enclosing 'p' is 'apid'.
'v'+'apid'='VAPID'
'in' acts as a link.
(Other definitions for vapid that I've seen before include "Dull and boring" , "no interest" , "Dull - uninteresting" , "Lacking flavour" , "Boring or lifeless" .)