Charles and Mike, it seems, are in a big hole (5)
I believe the answer is:
chasm
'a big hole' is the definition.
(I've seen this in another clue)
'charles and mike' is the wordplay.
'charles' becomes 'Chas' (shortened form).
'and' says to put letters next to each other.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'chas'+'m'='CHASM'
'it seems are in' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for chasm that I've seen before include "Abyss" , "Deep gap" , "Deep fissure; gulf" , "Great divide" , "rent" .)