Traps placed outside small dens (5)
I believe the answer is:
nests
'dens' is the definition.
(I've seen this in another clue)
'traps placed outside small' is the wordplay.
'traps' becomes 'nets' (net is a kind of trap).
'placed outside' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'nets' placed around 's' is 'NESTS'.
(Other definitions for nests that I've seen before include "Eyries" , "Birds' homes" , "Places to rear the young?" , "Structures for laying eggs" , "Bird houses" .)