Short versions of it are held by cities' chief crier (6)

'short versions' is the definition.
I know nothing about this answer so I can't tell whether this works.

'of it are held by cities' chief crier' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'held by' means one lot of letters goes inside another.
'cities' chief crier' becomes 'ths' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it'+'a'='ita'
'ita' is an anagram of 'ati'.
'ati' put into 'ths' is 'THATIS'.

Can you help me to learn more?