It's glee, strangely, shown by a mournful poet (7)
I believe the answer is:
elegist
'a mournful poet' is the definition.
(I've seen this before)
'it's glee strangely' is the wordplay.
'strangely' is an anagram indicator.
'its'+'glee'='itsglee'
'itsglee' with letters rearranged gives 'ELEGIST'.
'shown by' is the link.
(Other definitions for elegist that I've seen before include "Kind of writer" , "Poet, e.g. Gray" , "Writer of reflective poems" , "Mournful poet" , "bemoaning linesman" .)