Pensive, will shortly set about weird stuff, mostly (7)
I believe the answer is:
wistful
'pensive' is the definition.
(synonyms)
'will shortly set about weird stuff mostly' is the wordplay.
'shortly' means to remove the last letter.
'set about' means one lot of letters goes inside another.
'weird' is an anagram indicator.
'mostly' means to remove the last letter.
'will' with its final letter taken away is 'wil'.
'stuff' with its last letter removed is 'stuf'.
'stuf' with letters rearranged gives 'stfu'.
'wil' enclosing 'stfu' is 'WISTFUL'.
(Other definitions for wistful that I've seen before include "(Of a smile) showing vague or regretful longing" , "Sadly pensive" , "Melancholy" , "Regretfully longing" , "Disconsolate" .)