Stiffly formal power side (4)

I believe the answer is:

prim

'stiffly formal' is the definition.
(prim can mean proper or formal)

'power side' is the wordplay.
'power' becomes 'p' (symbol in physics).
'side' becomes 'rim' (rim is a kind of side).
'p'+'rim'='PRIM'

(Other definitions for prim that I've seen before include "Old-maidish" , "Straitlaced and proper" , "Affectedly formal" , "Prudishly disapproving" , "Proper or prissy" .)

I've seen this clue in the Sydney Morning Herald.

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