In the front I erected a row of seats (4)
I believe the answer is:
tier
'a row of seats' is the definition.
(I've seen this before)
'in the front i erected' is the wordplay.
'in' indicates a hidden word.
'TIER' can be found hidden inside 'front i erected'.
(Other definitions for tier that I've seen before include "Plague" , "Bank - row" , "Row in theatre" , "band" , "Level of a structure" .)