One politician, old Trump, explodes spontaneously (9)
I believe the answer is:
impromptu
'spontaneously' is the definition.
(synonyms)
'one politician old trump explodes' is the wordplay.
'one' becomes 'i' (Roman numeral).
'politician' becomes 'mp' (Member of Parliament).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'explodes' indicates an anagram.
'o'+'trump'='otrump'
'otrump' is an anagram of 'romptu'.
'i'+'mp'+'romptu'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Spontaneous" , "On the spur of the moment" , "Without pre-planning" , "Without much planning" , "Without rehearsal or preparation" .)