Mischievous imp (7)
(Other definitions for gremlin that I've seen before include "Fault-inducing sprite" , "Mischievous elf (in aircraft?)" , "Troublemaking goblin" , "Mischievous machine saboteur" , "Impish troublemaker" .)
'mischievous imp' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hobgoblin that I've seen before include "Ugly elf" , "Mischievous sprite - bugbear" , "bogey" , "Evil creature" , "A mischievous or evil spirit" .)