Charlie's copying Mike in holidaying under canvas (7)
I believe the answer is:
camping
'holidaying under canvas' is the definition.
(tents are made of canvas)
'charlie's copying mike in' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'copying' becomes 'aping' ('ape' can be a synonym of 'copy').
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'aping' going around 'm' is 'amping'.
'c'+'amping'='CAMPING'
(Other definitions for camping that I've seen before include "appearing affected?" , "Living in a tent" , "leisure activity" , "Taking temporary shelter" , "Type of holiday" .)