Reliability: function supplied by stronghold (13)
I believe the answer is:
steadfastness
'reliability' is the definition.
(thesaurus)
'function supplied by stronghold' is the wordplay.
'function' becomes 'stead' (stead is a kind of function).
'supplied by' means one lot of letters go next to another.
'stronghold' becomes 'fastness' (I've seen this in another clue).
'stead'+'fastness'='STEADFASTNESS'
(Other definitions for steadfastness that I've seen before include "Loyalty" , "Resolute attitude" , "Firm unwavering quality" , "Resolution" .)