Hard, taking hour from start to finish (7)
I believe the answer is:
through
'from start to finish' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I can't see?
'hard taking hour' is the wordplay.
'hard' becomes 'tough' (synonyms).
'taking' indicates putting letters inside.
'hour' becomes 'hr' (abbreviation).
'tough' placed around 'hr' is 'THROUGH'.
Can you help me to learn more?
(Other definitions for through that I've seen before include "Finished; by way of" , "Per" , "done" , "By means of; having finished a relationship" , "As a result of" .)