Resentful unknown shuns enthusiastic Jilly’s tip for starters (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

J <illy> (“tip” means first letter only; “for starters” means comes at the front of the word) + <z> EALOUS (=enthusiastic; “ unknown (=z, in algebra) shuns” means letter “z” is dropped)