A grate leaving a small gap? (4)
I believe the answer is:
ajar
'leaving a small gap?' is the definition.
(as in a door left slightly open or ajar)
'a grate' is the wordplay.
'grate' becomes 'jar' (both can mean to irritate or annoy).
'a'+'jar'='AJAR'
(Other definitions for ajar that I've seen before include "Part-open" , "not slammed shut" , "near closing?" , "Partially open" , "A litle bit open" .)