Outsize builder's ledge round room for electronic instrument (12)

'instrument' is the definition.
(I know that oscilloscope is a type of instrument)

'outsize builder's ledge round room' is the wordplay.
'outsize' becomes 'os'.
'builder's ledge' becomes 'cill' (I can't explain this - if you can you should believe this answer much more).
'round' becomes 'o' (round can mean a round object).
'room' becomes 'scope' (as in scope or room for manoeuvre).
'os'+'cill'+'o'+'scope'='OSCILLOSCOPE'

'for electronic' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?