Designed average central heating (5)
I believe the answer is:
meant
'designed' is the definition.
(thesaurus)
'average central heating' is the wordplay.
'average' becomes 'mean' (type of average).
'central' indicates the central letters.
The central letter of 'heating' is 't'.
'mean'+'t'='MEANT'
(Other definitions for meant that I've seen before include "Deliberate" , "Had as intention or purpose" , "planned" , "Implied" , "proposed" .)