Fellow mediums with proficiency (7)
I believe the answer is:
command
'proficiency' is the definition.
(thesaurus)
'fellow mediums with' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'mediums' becomes 'mm' (two M's - M means Medium as in clothing sizes).
'with' becomes 'and'.
'co'+'mm'+'and'='COMMAND'
(Other definitions for command that I've seen before include "Understanding" , "Bid" , "Instruction, order" , "Authoritative order" , "Authoritative instruction" .)