Assiduously, tho’ harshly (10)

I believe the answer is:

thoroughly

I'm a little stuck... Click here to teach me more about this clue!

'assiduously' is the definition.
(thesaurus)

'tho' harshly' is the wordplay.
'harshly' becomes 'roughly' (I can't explain this - if you can you should believe this answer much more).
'tho'+'roughly'='THOROUGHLY'

Can you help me to learn more?

(Other definitions for thoroughly that I've seen before include "Musical with Julie Andrews" , "Completely" , "More than a bit" , "Quite" , "Really" .)

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app