Doubt American power's constrained by wisdom (8)
I believe the answer is:
suspense
'doubt' is the definition.
(suspense is a kind of doubt)
'american power's constrained by wisdom' is the wordplay.
'american' becomes 'us'.
'power' becomes 'p'.
'constrained by' is an insertion indicator.
'wisdom' becomes 'sense' (sense is a kind of wisdom).
'us'+'p'='usp'
'usp' put inside 'sense' is 'SUSPENSE'.
(Other definitions for suspense that I've seen before include "State of anxiety or excitement" , "Discontinuance - indecision - excitement" , "Hitchcock's forte" , "Uncertain state" , "State of nervous uncertainly" .)