Because some effort is lacking (3)
I believe the answer is:
for
'because' is the definition.
(I've seen this before)
'effort is lacking' is the wordplay.
'FOR' is hidden in amongst the letters.
However, I'm not sure how this is indicated.
'some' acts as a 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" .)