To use one's brain less than fully is poor (4)
I believe the answer is:
thin
'poor' is the definition.
(I've seen this in other clues)
'to use one's brain less than fully' is the wordplay.
'to use one's brain' becomes 'think' (you use your brain to think).
'less than fully' means to remove the last letter.
'think' with its last letter taken off is 'THIN'.
'is' is the link.
(Other definitions for thin that I've seen before include "Slender, narrow" , "Made of light material" , "Narrow, slim" , "Measure" , "Flimsy" .)