Grind having hollowed out green tree (5)
I believe the answer is:
gnash
'grind' is the definition.
(as in gnashing or grinding one's teeth)
'hollowed out green tree' is the wordplay.
'hollowed out' means to remove the middle letters.
'tree' becomes 'ash' (ash is a kind of tree).
'green' with its middle removed is 'gn'.
'gn'+'ash'='GNASH'
'having' acts as a link.
(Other definitions for gnash that I've seen before include "grate" , "champ" , "grinding action" , "Strike one's teeth together in rage" , "Grind one's teeth" .)