Now and then spend up to visit old city’s ruins (8)
I believe the answer is:
destroys
'ruins' is the definition.
(to destroy is to ruin or break)
'now and then spend up to visit old city's' is the wordplay.
'now and then' indicates alternate letters (letters taken on a regular basis, now and then).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to visit' is a charade indicator (letters next to each other).
'old city's' becomes 'Troy's' (ancient city).
The alternate letters of 'spend' are 'sed'.
'sed' back-to-front is 'des'.
'des'+'troys'='DESTROYS'
(Other definitions for destroys that I've seen before include "Demolishes, ruins" , "Spoils or breaks completely" , "Puts down" , "Spoils or ruins completely" , "Spoils completely, perhaps kills" .)