Urge principal to be intellectual (7)
I believe the answer is:
egghead
'be intellectual' is the definition.
(I've seen this before)
'urge principal' is the wordplay.
'urge' becomes 'egg' (as in egging someone on).
'principal' becomes 'head' (synonyms).
'egg'+'head'='EGGHEAD'
'to' is the link.
(Other definitions for egghead that I've seen before include "where soldiers may take a dip?" , "Intellectual (colloq.)" , "Brainiac" , "Informal intellectual of ovoid type" , "Expert" .)