Old MP hugs excited girl, one aboard merry-go-round (9)
I believe the answer is:
whirligig
'merry-go-round' is the definition.
(synonyms)
'old mp hugs excited girl one aboard' is the wordplay.
'old mp' becomes 'whig' (I've seen this before).
'hugs' means one lot of letters goes inside another.
'excited' is an anagram indicator.
'one' becomes 'I' (Roman numeral).
'aboard' is an insertion indicator.
'girl' is an anagram of '[girl]'.
'whig' placed around '[girl]' is 'whi[girl]g'.
'whirlgig' going around 'i' is 'WHIRLIGIG'.
(Other definitions for whirligig that I've seen before include "Spinning toy - dizzying events" , "Spinner" , "Spinning top - water beetle" , "Carousel, merry-go-round" , "Spinning-toy" .)