Because some effort is lacking (3)
I believe the answer is:
for
'because' is the definition.
(I've seen this in other clues)
'effort is lacking' is the wordplay.
'FOR' is hidden in amongst the letters.
But, I am uncertain how the hidden word is indicated.
'some' is the link.
Can you help me to learn more?
(Other definitions for for that I've seen before include "Approving" , "Favouring" , "In favour of; since" , "and 16 and 20: Notwithstanding" , "Pro" .)