Pretty model T-rex seen by me in Cambridgeshire city (9)
I believe the answer is:
extremely
'pretty' is the definition.
(thesaurus)
'model t-rex seen by me in cambridgeshire city' is the wordplay.
'model' indicates anagramming the letters.
'seen by' is a charade indicator (letters next to each other).
'in' is an insertion indicator.
'cambridgeshire city' becomes 'ely' (I've seen this before).
't' put after 'rex' is 'rext'.
'rext'+'me'='rextme'
'rextme' anagrammed gives 'xtreme'.
'xtreme' going within 'ely' is 'EXTREMELY'.
(Other definitions for extremely that I've seen before include "To a high degree or extent" , "remarkably" .)