A conservative old American jerk with no power (8)
I believe the answer is:
acoustic
'no power' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.
'a conservative old american jerk' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us' (abbreviation for United States).
'jerk' becomes 'tic' (jerking movement by the body).
'a'+'c'+'o'+'us'+'tic'='ACOUSTIC'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for acoustic that I've seen before include "dispensing with amps?" , "Like sound" , "Relating to sound" , "Of the physical properties of sound" , "This guitar is not electric" .)