Implied temperature is below average (5)
I believe the answer is:
meant
'implied' is the definition.
(imply can mean to mean something)
'temperature is below average' is the wordplay.
'temperature' becomes 't' (abbreviation).
'is below' means one lot of letters go next to another (in a down clue, some letters go below others).
'average' becomes 'mean' (type of average).
't' after 'mean' is 'MEANT'.
(Other definitions for meant that I've seen before include "Intentional" , "Implied" , "no accident" , "Aimed" , "Deliberate" .)