Extremely smart band with guitar part (6)
I believe the answer is:
string
'guitar part' is the definition.
(I've seen this in another clue)
'extremely smart band' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'band' becomes 'ring' (a ring is a band of metal).
'smart' with its centre taken out is 'st'.
'st'+'ring'='STRING'
'with' is the link.
(Other definitions for string that I've seen before include "Group of horses" , "Sequence; cord" , "Violin wire/catgut" , "Violin wire" , "Number of animals" .)