Hawkwind EP that's extensive (2,5)
I believe the answer is:
in depth
'extensive' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'hawkwind ep' is the wordplay.
I cannot quite understand how this works, but
'ep' is within the answer.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for in depth that I've seen before include "Swimmer so able to touch bottom" , "Thorough" , "Detailed sort of investigation" , "Thoroughly and comprehensively" , "comprehensive" .)