Second US president back in A&E — cause of dysentery? (7)
I believe the answer is:
amoebae
'cause of dysentery?' is the definition.
(as in amoebic dysentery)
'second us president back in ae' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'us president' becomes 'Abe' (Abraham (Abe) Lincoln).
'back' says the letters should be written backwards.
'in' is an insertion indicator.
'abe' written backwards gives 'eba'.
'mo'+'eba'='moeba'
'moeba' placed inside 'ae' is 'AMOEBAE'.
(Other definitions for amoebae that I've seen before include "Single-celled animals" , "Protozoans" , "Unicellular organisms" .)