Because some effort is lacking (3)

I believe the answer is:

for

'because' is the definition.
(thesaurus)

'some effort is lacking' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'FOR' is hidden within 'effort lacking'.

(Other definitions for for that I've seen before include "Approving" , "Favouring" , "In favour of; since" , "and 16 and 20: Notwithstanding" , "Pro" .)

I've seen this clue in the Evening Standard.

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