'Bubbles' mostly must be Turner (5)
I believe the answer is:
lathe
'turner' is the definition.
(a lathe is something which 'turns')
'bubbles mostly' is the wordplay.
'bubbles' becomes 'lather' (I've seen this before).
'mostly' means to remove the last letter.
'lather' with its last letter removed is 'LATHE'.
'must be' acts as a link.
(Other definitions for lathe that I've seen before include "Machine that turns wood or metal for cutting" , "Wood-turning machine" , "Turner's tool" , "This is turned out" , "Machine for cutting, drilling or polishing" .)