Slight guess, inconclusive (4)
I believe the answer is:
thin
'slight' is the definition.
('thin' can be similar in meaning to 'slight')
'guess inconclusive' is the wordplay.
'guess' becomes 'think' ('think' can be a synonym of 'guess').
'inconclusive' means to remove the last letter.
'think' with its last letter removed is 'THIN'.
(Other definitions for thin that I've seen before include "Slender, slight" , "Lean; narrow" , "With little thickness" , "Spare" , "Having little width" .)