Pattern in sky? Builder’s installed city’s first elevated railway (8)
I believe the answer is:
mackerel
'pattern in sky?' is the definition.
(mackerel sky can mean a cloudy sky)
'builder's installed city's first elevated railway' is the wordplay.
'builder' becomes 'maker'.
'installed' means one lot of letters goes inside another.
'city's first' becomes 'c' (1st letter of 'city').
'elevated railway' becomes 'el' (synonyms).
'maker' enclosing 'c' is 'macker'.
'macker'+'el'='MACKEREL'
(Other definitions for mackerel that I've seen before include "Sea-fish" , "Food fish related to tuna" , "Commonly-eaten fish" , "Fish caught by spinner (or sprat?)" , "One in school" .)