Darkest shadow observed when umbrella loses length (5)
I believe the answer is:
umbra
'darkest shadow' is the definition.
'umbra' can be an answer for 'shadow' (umbra is a kind of shadow). I'm unsure of the 'darkest' bit.
'umbrella loses length' is the wordplay.
'loses' indicates named letters should be taken away.
'length' becomes 'ell' (I've seen this before).
'umbrella' with 'ell' taken away is 'UMBRA'.
'observed when' acts as a link.
Can you help me to learn more?
(Other definitions for umbra that I've seen before include "Full shadow" , "Burma (anag.)" , "Shade, or ghost" , "Rumba (anag.)" , "Region of shadow" .)