In that case it will be hard within an hour (4)
I believe the answer is:
then
'in that case' is the definition.
(synonyms)
'it will be hard within an hour' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found in the answer.
The remaining letters 'en' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for then that I've seen before include "Subsequently - previously" , "At that time; next" , "Once upon a time" , "Consequently" , "Afterwards - as a result" .)