Grub in tea urn, oddly, and a musical instrument! (6)
I believe the answer is:
guitar
'a musical instrument' is the definition.
(I know that Guitar is a musical instrument)
'grub in tea urn oddly' is the wordplay.
'oddly' indicates alternate letters (letters in odd-numbered positions).
'grub'+'in'+'tea'+'urn'='grubinteaurn'
The alternating letters of 'grubinteaurn' are 'GUITAR'.
'and' acts as a link.
(Other definitions for guitar that I've seen before include "Concern" , "Popular stringed instrument used by Beatles" , "One played with plectrum" , "Axe" , "Talk under one's breath" .)