Stronghold must be built with speed? (8)
I believe the answer is:
fastness
'stronghold' is the definition.
(I know that stronghold can be written as fastness)
'built with speed?' is the wordplay.
I cannot quite see how this works, but
'speed?' could be 'fast' (I've seen this before) and 'fast' is located in the answer.
The remaining letters 'ness' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'must be' acts as a link.
Can you help me to learn more?
(Other definitions for fastness that I've seen before include "speed?" , "Stronghold or fortress" , "Keep" , "Secure refuge" , "quality of not fading" .)