Be cool Charlie, take the high ground! (5)
I believe the answer is:
chill
'be cool' is the definition.
(similar in meaning)
'charlie take the high ground' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'take' says to put letters next to each other.
'the high ground' becomes 'hill' (I've seen this before).
'c'+'hill'='CHILL'
(Other definitions for chill that I've seen before include "Sensation of cold or dread" , "This disheartens" , "Preserve by cold" , "Unpleasent coldness in the air" , "Iciness" .)