A number caught up in online trading (3)
I believe the answer is:
ten
'a number' is the definition.
(ten is a number)
'caught up in online trading' is the wordplay.
'caught up' shows that the letters should be reversed in order (in a down clue, letters go up).
'in' indicates the answer is hidden within the clue.
'net' is hidden within 'online trading'.
'net' back-to-front is 'TEN'.
(Other definitions for ten that I've seen before include "Five times two" , "PM's address, Downing St" , "Base of decimal system" , "X-WORD" , "Cardinal number" .)