Instrument in the form of a tub (4)
I believe the answer is:
tuba
'instrument' is the definition.
(I know that tuba is a type of bass)
'form of a tub' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'a'+'tub'='atub'
'atub' with letters rearranged gives 'TUBA'.
'in the' is the link.
(Other definitions for tuba that I've seen before include "One's blown" , "Brass band instrument" , "Large low-pitched brass musical instrument" , "Large brass wind instrument" , "music-maker" .)