Living thing (northern one) in immoderate excitement (8)
I believe the answer is:
organism
'living thing' is the definition.
(synonyms)
'northern one in immoderate excitement' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'immoderate excitement' becomes 'orgasm' (sexual excitement).
'n'+'i'='ni'
'ni' inserted inside 'orgasm' is 'ORGANISM'.
(Other definitions for organism that I've seen before include "Living animal or plant" , "Individual life form" , "Body - cell - Mao grins (anag)" , "individual creation" , "Something alive" .)