Are there a number of them on one coin? (5)
I believe the answer is:
heads
'are there a number' is the definition.
I can't tell whether this definition defines the answer.
'them on one coin?' is the wordplay.
'them' becomes 'hes' (I am not sure about this - if you are sure you should believe this answer much more).
'on' indicates putting letters inside (as in clothing 'on' a person).
'one' becomes 'a' (a thing is one thing).
'coin?' becomes 'd' (old British penny abbreviation).
'a'+'d'='ad'
'hes' going around 'ad' is 'HEADS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for heads that I've seen before include "a 50-50 chance" , "Noddles; nuts" , "Goes for" , "Leaders" , "Obverse result of coin-tossing" .)