American power gripped by feeling apprehension (8)
I believe the answer is:
suspense
'apprehension' is the definition.
(suspense is a kind of apprehension)
'american power gripped by feeling' is the wordplay.
'american' becomes 'us'.
'power' becomes 'p'.
'gripped by' indicates putting letters inside.
'feeling' becomes 'sense' (I've seen this before).
'us'+'p'='usp'
'usp' put within 'sense' is 'SUSPENSE'.
(Other definitions for suspense that I've seen before include "Uncertain anticipation" , "Uneasy uncertainty" , "State of uncertainty about the future" , "Hitchcock's speciality" , "Tense expectation" .)