Tickle large prickly head (6)
I believe the answer is:
teasel
'prickly head' is the definition.
(plant with a prickly flowerhead)
'tickle large' is the wordplay.
'tickle' becomes 'tease'.
'large' becomes 'l'.
'tease'+'l'='TEASEL'
(Other definitions for teasel that I've seen before include "Plant with spiny purple flower heads" , "Sat Lee down on the spiny herb" , "Prickly plant (S, not Z)" , "Prickly plant - elates (anag)" , "it helps us to nap" .)