Show round a number in the depot (11)
I believe the answer is:
'show' is the definition.
(I've seen this before)
'round a number in the depot' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'a number' becomes 'ten' (ten is a number).
'in' indicates putting letters inside.
'the depot' becomes 'station' (I've seen this before).
'ten' inserted inside 'station' is 'stentation'.
(Other definitions for ostentation that I've seen before include "Pretentious display", "show", "Showiness".)