Those present are in it (10)
I believe the answer is:
attendance
'those present' is the definition.
(I've seen this before)
'are in it' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'tendance' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for attendance that I've seen before include "People present" , "Number at gathering" , "Number present" , "Act of being present" , "all that turned up" .)