Diffident about power issue (4)
I believe the answer is:
copy
'issue' is the definition.
(thesaurus)
'diffident about power' is the wordplay.
'diffident' becomes 'coy' (I've seen this in another clue).
'about' indicates putting letters inside.
'power' becomes 'p' (symbol in physics).
'coy' placed around 'p' is 'COPY'.
(Other definitions for copy that I've seen before include "Imitation or matter for printing" , "transcription" , "Fake" , "Material for publication" , "material for article?" .)