Firm to explain about computers, etc (8)
I believe the answer is:
definite
'firm' is the definition.
(thesaurus)
'explain about computers etc' is the wordplay.
'explain' becomes 'define' (I've seen this before).
'about' indicates putting letters inside.
'computers etc' becomes 'it' (short for Information Technology).
'define' enclosing 'it' is 'DEFINITE'.
'to' acts as a link.
(Other definitions for definite that I've seen before include "Clearly decided" , "Sure, positive" , "Clearly marked out" , "Unmistakable" , "Certain; clearly stated" .)