Short month in which temperature rises a degree (4)
I believe the answer is:
step
'degree' is the definition.
(I've seen this in another clue)
'short month in which temperature' is the wordplay.
'short month' becomes 'Sep' (abbreviation for September; 'short' indicates a short form).
'in which' means one lot of letters goes inside another.
'temperature' becomes 't' (abbreviation).
'sep' enclosing 't' is 'STEP'.
'rises a' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for step that I've seen before include "Stair" , "Rung, dance movement" , "Footfall" , "some progress" , "Measure taken" .)