Bob’s one hard song to perform (6)
I believe the answer is:
hairdo
'bob's' is the definition.
(bob is a type of hairdo)
'one hard song to perform' is the wordplay.
'one hard' becomes 'h' (abbreviation used in pencil classifications).
'song' becomes 'air' (I've seen this in another clue).
'to perform' becomes 'do' (both can mean complete an action).
'h'+'air'+'do'='HAIRDO'
(Other definitions for hairdo that I've seen before include "Bob, perm or bouffant" , "that may go with a bang?" , "capital arrangement" , "What one may have expected from 'Sweeney Todd'" , "Pageboy maybe" .)