Extent in which the morning has only a part (5)
I believe the answer is:
ambit
'extent' is the definition.
(ambit is a kind of extent)
'the morning has only a part' is the wordplay.
'the morning' becomes 'am' (as in bi:00 AM).
'has' is a charade indicator (letters next to each other).
'only a part' becomes 'bit' (I can't justify this - if you can you should believe this answer much more).
'am'+'bit'='AMBIT'
'in which' acts as a link.
Can you help me to learn more?
(Other definitions for ambit that I've seen before include "Compass, scope" , "round" , "Scope, extent, or bounds of something" , "Range, scope" , "Scope or confines" .)