The fellow's getting hold of doctor, one so upset and sweating (8)

'sweating' is the definition.
(I know that sweating can be written as hidrosis)

'the fellow's getting hold of doctor one so upset' is the wordplay.
'the fellow's' becomes 'his' (belonging to some man or fellow).
'getting hold of' means one lot of letters goes inside another.
'doctor' becomes 'Dr' (abbreviation).
'one so' becomes 'iso' (I can't explain this - if you can you should believe this answer much more).
'upset' shows that the letters should be reversed in order.
'iso' reversed gives 'osi'.
'dr'+'osi'='drosi'
'his' enclosing 'drosi' is 'HIDROSIS'.

'and' acts as a link.

Can you help me to learn more?