Fairly scorching at first, temperature in summer month (6)

'fairly' is the definition.
(thesaurus)

'scorching at first temperature in summer month' is the wordplay.
'at first' indicates taking the first letters.
'temperature' becomes 't' (abbreviation).
'in' means one lot of letters goes inside another.
'summer month' becomes 'july' (I've seen this before).
The initial letter of 'scorching' is 's'.
's'+'t'='st'
'st' inserted within 'july' is 'JUSTLY'.