Instrument is British with extremely unusual enamel engraving on top (7)
I believe the answer is:
ukulele
'instrument' is the definition.
(ukulele is a kind of instrument)
'british with extremely unusual enamel engraving on top' is the wordplay.
I cannot quite understand how this works, but
'british' could be 'uk' (I've seen this in other clues) and 'uk' is found in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for ukulele that I've seen before include "Musical instrument once played by George Formby" , "Small 4-stringed guitar" , "One needs pluck in this" , "Small type of guitar" , "Small Hawaiian guitar" .)