Mounting attack, a hero runs and runs (9)

'runs' is the definition.
(diarrhoea is informally called 'the runs')

'mounting attack a hero runs' is the wordplay.
'mounting' says the letters should be written backwards (going up in a down clue).
'attack' becomes 'raid' (raiding is a kind of attacking).
'runs' indicates anagramming the letters.
'raid' backwards is 'diar'.
'a'+'hero'='ahero'
'ahero' anagrammed gives 'rhoea'.
'diar'+'rhoea'='DIARRHOEA'

'and' is the link.