Plants whistle-blower to infiltrate works (8)
I believe the answer is:
trefoils
'plants' is the definition.
(trefoil is a kind of plant)
'whistle-blower to infiltrate works' is the wordplay.
'whistle blower' becomes 'ref' (I've seen this in other clues).
'to infiltrate' means one lot of letters goes inside another.
'works' becomes 'toils' (toil is a kind of work).
'ref' put within 'toils' is 'TREFOILS'.
(Other definitions for trefoils that I've seen before include "Plants such as clover" , "Leaves divided into three leaflets" , "Plants with three-lobed leaves" , "Tiles for a change to shamrocks, say" .)