Depression becomes difficult when temperature drops (6)
I believe the answer is:
trough
'depression' is the definition.
(I know that trough is a type of depression)
'difficult when temperature drops' is the wordplay.
I cannot quite understand how this works, but
'difficult' could be 'rough' (similar in meaning) and 'rough' is located in the answer.
'temperature' could be 't' (abbreviation) and 't' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'becomes' acts as a link.
Can you help me to learn more?
(Other definitions for trough that I've seen before include "Narrow area of low pressure" , "Depression - animal food holder" , "period when little is achieved" , "Hollow between waves" , "Container for cattle-feed" .)