Alchemi needs large cinema organ (8)
I believe the answer is:
melodeon
'organ' is the definition.
(thesaurus)
'alchemi needs large cinema' is the wordplay.
'alchemi' becomes 'me' (where the crossword setter is 'Alchemi').
'needs' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'cinema' becomes 'odeon' (I've seen this before).
'me'+'l'+'odeon'='MELODEON'
(Other definitions for melodeon that I've seen before include "Small squeezebox" , "A kind of accordion" , "music-maker" , "A small accordion" , "Musical instrument" .)