MP’s niece surprisingly becoming a model (8)
I believe the answer is:
specimen
'a model' is the definition.
(thesaurus)
'mp's niece surprisingly' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'mps'+'niece'='mpsniece'
'mpsniece' with letters rearranged gives 'SPECIMEN'.
'becoming' acts as a link.
(Other definitions for specimen that I've seen before include "Representative sample" , "Part taken as example of whole or class" , "Men's epic (anag.)" , "Exemplar" , "Sample taken for medical testing" .)