Might at last trap someone in red? (5)
I believe the answer is:
power
'might' is the definition.
(being mighty)
'last trap someone in red?' is the wordplay.
'last' says to take the final letters.
'someone in red?' becomes 'ower' (being in the red means owing money).
The last letter of 'trap' is 'p'.
'p'+'ower'='POWER'
'at' is the link.
(Other definitions for power that I've seen before include "Rate of doing work" , "product" , "Strength, force" , "Strength - capacity" , "Strength or authority" .)