Stick close to flatmate in block (4)
I believe the answer is:
bear
'stick' is the definition.
(thesaurus)
'close to flatmate in block' is the wordplay.
'close to' indicates one should take the final letters (the 'close' to the word).
'in' indicates putting letters inside.
'block' becomes 'bar' (block can mean to ban or bar).
The last letter of 'flatmate' is 'e'.
'e' put inside 'bar' is 'BEAR'.
(Other definitions for bear that I've seen before include "stand" , "eg Pooh" , "grizzly, perhaps?" , "Brook" , "Support, large mammal" .)