Number seen with tennis pants after smalls have been discarded (8)
I believe the answer is:
nineteen
'number' is the definition.
(I know that nineteen is a octagonal number)
I don't understand the remainder of the clue.
(Other definitions for nineteen that I've seen before include "12 + 7" , "after the round, the number for the next round?" , "Prime number" , "19" , "This clue's number" .)