The top's come off loudspeaker -- bother! (5)
I believe the answer is:
annoy
'bother' is the definition.
(I know that bother can be written as annoy)
'the top's come off loudspeaker' is the wordplay.
'the top's come off' means to remove the first letter.
'loudspeaker' becomes 'tannoy' (tannoy is a kind of loudspeaker).
'tannoy' with its initial letter removed is 'ANNOY'.
(Other definitions for annoy that I've seen before include "be a nuisance" , "Bother, irritate" , "Bug, rile" , "Nark - nettle" , "Irk" .)