Sensible friend grasps your abbreviation (6)
I believe the answer is:
mature
'sensible' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mature that I've seen before include "Fully-developed" , "Ripe or fully grown" , "Age" , "sensible" , "due to be paid" .)