Old lag setting fire to plant (7)
I believe the answer is:
conifer
'plant' is the definition.
The answer is an example.
'old lag setting fire' is the wordplay.
'old lag' becomes 'con' (both can mean a convict. I am not sure about the 'old' bit.).
'setting' indicates an anagram.
'fire' with letters rearranged gives 'ifer'.
'con'+'ifer'='CONIFER'
'to' is the link.
Can you help me to learn more?
(Other definitions for conifer that I've seen before include "The tree doesn't lose its leaves" , "Fir or larch, for example" , "Tree such as pine, yew etc" , "Type of tree" , "Fir or pine, for example" .)