Do I hear a tail-ender finally bats, getting the runs? (9)

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

'do i hear a tail-ender finally bats' is the wordplay.
'tail' indicates one should take the final letters.
'finally' says to put letters next to each other (some letters go at the end of others).
'bats' indicates an anagram (slang for mad).
The final letter of 'ender' is 'r'.
'do'+'i'+'hear'+'a'+'r'='doihearar'
'doihearar' is an anagram of 'DIARRHOEA'.

'getting' acts as a link.