Charles and Mike together fill the gap (5)
I believe the answer is:
chasm
'the gap' is the definition.
(chasm is a kind of gap)
'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'
'together fill' is the link.
I am not very happy about this link. 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 "Gaping hollow" , "Pit" , "Abyss" , "Wide gap" , "One yawns" .)