He lays slates, 50 in a row (5)
I believe the answer is:
tiler
'he lays slates' is the definition.
(a tiler lays tiles or slates)
'50 in a row' is the wordplay.
'50' becomes 'l' (Roman numeral).
'in' indicates putting letters inside.
'a row' becomes 'tier' (a tier is a row of things).
'l' inserted inside 'tier' is 'TILER'.
(Other definitions for tiler that I've seen before include "Tradesman who covers roofs or bathroom walls with rectangular materials" , "Craftsman" , "Roof worker" , "doorkeeper at lodge" , "Working man" .)