The craft with which Spain built up her empire (7)
I believe the answer is:
galleon
'the craft' is the definition.
(type of ship)
'spain built up her empire' is the wordplay.
I cannot quite understand how this works, but
'spain' could be 'e' (abbreviation for Espana) and 'e' is found within the answer.
The remaining letters 'gallon' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'with which' acts as a link.
Can you help me to learn more?
(Other definitions for galleon that I've seen before include "One might sink" , "Large Spanish sailing ship of yore" , "Old Spanish war-ship" , "Spanish treasure ship" , "Trader" .)