Hundreds of them form a unit (5)
I believe the answer is:
cents
'hundreds' is the definition.
The answer and definition can be both quantities as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
'them form a unit' is the wordplay.
I cannot really understand how this works, but
'unit' could be 't' (T is an example) and 't' is found in the answer.
The remaining letters 'cens' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for cents that I've seen before include "money abroad" , "American coins" , "Hundredths of dollar" , "Fractions of dollar" , "US money, e.g." .)