Reinforce stern Ghent building (10)
I believe the answer is:
strengthen
'reinforce' is the definition.
(I've seen this before)
'stern ghent building' is the wordplay.
'building' indicates an anagram.
'stern'+'ghent'='sternghent'
'sternghent' anagrammed gives 'STRENGTHEN'.
(Other definitions for strengthen that I've seen before include "Fortify" , "Build up" , "Toughen up" , "Reinforce, substantiate" , "Make tough" .)