Snakes, say, we spot regularly in nettles (8)
I believe the answer is:
reptiles
'snakes say' is the definition.
(I know that snake is a type of reptiles)
'we spot regularly in nettles' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'in' indicates putting letters inside.
'nettles' becomes 'riles' ('rile' can be a synonym of 'nettle').
'we'+'spot'='wespot'
The alternating letters of 'wespot' are 'ept'.
'ept' going inside 'riles' is 'REPTILES'.
(Other definitions for reptiles that I've seen before include "Snakes, lizards etc." , "Cold-blooded vertebrates" .)