Check loch -- this writer is on it (5)
I believe the answer is:
limit
'check' is the definition.
(limit can mean to check or restrain)
'loch this writer is on it' is the wordplay.
'loch' becomes 'l'.
'this writer is' becomes 'I'm' (I am - referring to the crossword setter).
'on' means one lot of letters go next to another.
'l'+'im'+'it'='LIMIT'
(Other definitions for limit that I've seen before include "maximum number" , "Ration" , "Ceiling" , "Keep in" , "Boundary, demarcation line" .)