Net split from below (4)
I believe the answer is:
trap
'net' is the definition.
(thesaurus)
'split from below' is the wordplay.
'split' becomes 'part' (synonyms).
'from below' shows that the letters should be reversed in order (in a down clue, letters go up).
'part' in reverse letter order is 'TRAP'.
(Other definitions for trap that I've seen before include "that may catch a speeding motorist" , "Snare - mouth" , "Means of transport (sometime sprung!)" , "get in net" , "mug" .)