Companion in apartment lifeless, getting Mike worried (8)
I believe the answer is:
flatmate
'companion in apartment' is the definition.
'lifeless getting mike worried' is the wordplay.
'lifeless' becomes 'flat'.
'getting' says to put letters next to each other.
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'worried' becomes 'ate' (something worrying may eat someone up).
'flat'+'m'+'ate'='FLATMATE'
(Other definitions for flatmate that I've seen before include "in the rooms one shares" , "one sharing same pad" , "Sharer of accommodation" , "Cohabitee" , "Person sharing accommodation" .)