On the front, extremely large, mature tree (3)
I believe the answer is:
elm
'tree' is the definition.
(elm is a kind of tree)
'on the front extremely large mature' is the wordplay.
'on the front' suggests taking the first letters.
The first letters of 'extremely large mature' is 'ELM'.
(Other definitions for elm that I've seen before include "Gray's rugged tree (Elegy)" , "one has a trunk and leaves" , "Kind of 19" , "Tree for a Cockney tiller" , "One [TYPE OF TREE]" .)