Tackle landlord about source of infestation (5)
I believe the answer is:
hoist
'tackle' is the definition.
(I've seen this before)
'landlord about source of infestation' is the wordplay.
'landlord' becomes 'host'.
'about' is an insertion indicator.
'source of' indicates taking the first letters.
The initial letter of 'infestation' is 'i'.
'host' going around 'i' is 'HOIST'.
(Other definitions for hoist that I've seen before include "Lift (mechanically)" , "Raise or haul up, say flag" , "Device for lifting, often attached to 5 down" , "set of flags" , "Heave upwards" .)