Gull lands on bit of krill it smells (4)
I believe the answer is:
conk
'it smells' is the definition.
('conk' is an informal term for nose)
'gull lands on bit of krill' is the wordplay.
'gull' becomes 'con' (both can mean to trick someone).
'lands on' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'bit of' suggests taking the first letters.
The initial letter of 'krill' is 'k'.
'con'+'k'='CONK'
(Other definitions for conk that I've seen before include "Nose (colloq.)" , "Blow" , "Hooter" , "Nose; break down" , "Punch" .)