The firm is old, surprisingly (5)
I believe the answer is:
solid
'the firm' is the definition.
(thesaurus)
'is old surprisingly' is the wordplay.
'surprisingly' is an anagram indicator.
'is'+'old'='isold'
'isold' is an anagram of 'SOLID'.
(Other definitions for solid that I've seen before include "Dense" , "Unbroken" , "Firm - sound - reliable" , "Concrete or reilable" , "Not hollow or liquid" .)