Gas constant? Age going up? Relax! (5)
I believe the answer is:
remit
'relax' is the definition.
(I've seen this before)
'gas constant? age going up?' is the wordplay.
'gas constant?' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'age' becomes 'time' (an age is a period of time).
'going up?' says the letters should be written in reverse (in a down clue, the letters are written upwards).
'time' written backwards gives 'emit'.
'r'+'emit'='REMIT'
Can you help me to learn more?
(Other definitions for remit that I've seen before include "Transfer (money)" , "Job specification" , "Defer" , "Send (money) in payment; pardon" , "Pay for timer" .)