Nick overheard beginning to design strange instrument (5,4)
I believe the answer is:
steel drum
'instrument' is the definition.
(type of drum)
'nick overheard beginning to design strange' is the wordplay.
I cannot quite understand how this works, but
'strange' could be 'rum' (similar in meaning) and 'rum' is found within the answer.
This explanation may well be incorrect...
(Other definitions for steel drum that I've seen before include "Trinidadian instrument" , "Metal instrument" .)