Old politician in real trouble? That's ideal! (8)
I believe the answer is:
exemplar
'ideal' is the definition.
(exemplar is a kind of ideal)
'old politician in real trouble?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'politician' becomes 'MP' (Member of Parliament).
'in' is an insertion indicator.
'trouble?' indicates an anagram (I've seen 'troubled' mean this).
'real' anagrammed gives 'elar'.
'mp' put into 'elar' is 'emplar'.
'ex'+'emplar'='EXEMPLAR'
'that's' acts as a link.
(Other definitions for exemplar that I've seen before include "A model or instance" , "Ideal model" , "Epitome" , "Model to be followed" , "Something serving as an appropriate model" .)