Reach a temperature that isn't regularly dropping (6)
I believe the answer is:
attain
'reach' is the definition.
(I know that reach can be written as attain)
'a temperature that isn't regularly dropping' is the wordplay.
'temperature' becomes 't' (abbreviation).
'regularly dropping' means one should take alternating letters (letters removed at regular intervals).
'that'+'isnt'='thatisnt'
The alternating letters of 'thatisnt' are 'tain'.
'a'+'t'+'tain'='ATTAIN'
(Other definitions for attain that I've seen before include "Succeed in reaching, achieve" , "Reach or gain" , "Reach objective or ambition" , "Reach objective, get there" , "Reach with effort" .)