An expert seizing power without delay (5)
I believe the answer is:
apace
'without delay' is the definition.
(I've seen this in another clue)
'an expert seizing power' is the wordplay.
'an' becomes 'a'.
'expert' becomes 'ace' (ace can informally mean very good).
'seizing' is an insertion indicator.
'power' becomes 'p' (symbol in physics).
'a'+'ace'='aace'
'aace' enclosing 'p' is 'APACE'.
(Other definitions for apace that I've seen before include "fast" , "(Move on) swiftly" , "What's quickly" , "at high speed" , "Swiftly, quickly" .)