Speed reached by the Spanish in very old metropolis (8)
I believe the answer is:
velocity
'speed' is the definition.
(I know that speed can be written as velocity)
'the spanish in very old metropolis' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'in' indicates putting letters inside.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'metropolis' becomes 'city' (a metropolis is a large city).
'v'+'o'='vo'
'el' placed inside 'vo' is 'velo'.
'velo'+'city'='VELOCITY'
'reached by' acts as a link.
(Other definitions for velocity that I've seen before include "Speed of travel" , "Rate of motion, distance per unit of time" , "Swiftness" , "Distance travelled per unit of time" , "Rate of movement." .)