Excuse company essentially about mobile phone message (7)
I believe the answer is:
pretext
'excuse' is the definition.
(a pretext is an excuse or pretence)
'company essentially about mobile phone message' is the wordplay.
'essentially' indicates the central letters.
'about' becomes 're' (regarding).
'mobile phone message' becomes 'text' (I've seen this before).
The middle letter of 'company' is 'p'.
'p'+'re'+'text'='PRETEXT'
(Other definitions for pretext that I've seen before include "Fictitious reason in order to conceal the real one" , "False reason as justification for an action" , "Alleged reason" , "Excuse for an action" , "False reason, guise" .)