Most of goo is thin (4)
I believe the answer is:
slim
'is thin' is the definition.
('slim' can be similar in meaning to 'thin')
'most of goo' is the wordplay.
'most of' means to remove the last letter.
'goo' becomes 'slime' ('slime' can be a synonym of 'goo').
'slime' with its last letter taken off is 'SLIM'.
(Other definitions for slim that I've seen before include "Lean" , "Slenderly built" , "to make one lose weight" , "Take off weight as by dieting" , "Bakery produce" .)