Very lengthy mature pine (7)
I believe the answer is:
agelong
'very lengthy' is the definition.
'agelong' can be an answer for 'lengthy' (similar in meaning). I am unsure of the 'very' bit.
'mature pine' is the wordplay.
'mature' becomes 'age' (synonyms).
'pine' becomes 'long' (to pine for something is to long for it).
'age'+'long'='AGELONG'
Can you help me to learn more?
(Other definitions for agelong that I've seen before include "Having existed for many a year" , "Seeming to have always been so" , "permanent" , "never going away?" , "Almost endless" .)