Surprisingly, disheartened janitor is hard worker (6)
I believe the answer is:
trojan
'is hard worker' is the definition.
(I've seen this before)
'surprisingly disheartened janitor' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'disheartened' says to hollow out the word (remove centre letters) (the middle or heart is removed).
'janitor' with its centre taken out is 'jantor'.
'jantor' with letters rearranged gives 'TROJAN'.
(Other definitions for trojan that I've seen before include "Work like a . . . . . ., with great energy and endurance - ask Helen" , "Dedicated worker" , "This horse was a wooden decoy" , "A citizen of ancient Troy" , "Subject of King Priam" .)