Alliance lacks power to make law (3)
I believe the answer is:
act
'law' is the definition.
(I've seen this in another clue)
'alliance lacks power' is the wordplay.
'alliance' becomes 'pact' (I've seen this in another clue).
'lacks' suggests deleting specific letters.
'power' becomes 'p' (symbol in physics).
'pact' with 'p' removed is 'ACT'.
'to make' acts as a link.
(Other definitions for act that I've seen before include "Accomplishment" , "Feat" , "Feign" , "Produce an effect" , "Function" .)