Pick ace out; it may relieve tension (3,4)
I believe the answer is:
ice pack
'it may relieve tension' is the definition.
The definition suggests a singular noun which matches the answer.
'pick ace out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'pick'+'ace'='pickace'
'pickace' is an anagram of 'ICE-PACK'.
Can you help me to learn more?
(Other definitions for ice pack that I've seen before include "It's applied to cool or reduce swelling" , "Cold item held against a swollen body part" , "This could reduce swelling" , "polar region" , "Cold bag for e.g. swelling" .)