Gas is thus rising GP admits (6)
I believe the answer is:
gossip
'gas' is the definition.
(I've seen this before)
'is thus rising gp admits' is the wordplay.
'thus' becomes 'so' (synonyms).
'rising' is a reversal indicator (going up in a down clue).
'admits' indicates putting letters inside.
'is'+'so'='isso'
'isso' backwards is 'ossi'.
'ossi' placed inside 'gp' is 'GOSSIP'.
(Other definitions for gossip that I've seen before include "Rabbit" , "Busybody" , "(Malicious) report about the doings of others" , "Idle and mildly malicious talk" , "Casual chit-chat" .)