Long piano in conservatoire at last (4)
I believe the answer is:
pine
'long' is the definition.
('pine' can be a synonym of 'long')
'piano in conservatoire at last' is the wordplay.
'piano' becomes 'p'.
'at last' indicates one should take the final letters.
The final letter of 'conservatoire' is 'e'.
'p'+'in'+'e'='PINE'
(Other definitions for pine that I've seen before include "Coniferous tree" , "Kind of needle" , "Thirst" , "Scots fir" , "Long for the conifer?" .)