A number departed in extremes of timidity (6)
I believe the answer is:
twenty
'a number' is the definition.
(twenty is a kind of number)
'departed in extremes of timidity' is the wordplay.
'departed' becomes 'went' (I've seen this in another clue).
'in' is an insertion indicator.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'timidity' with its middle removed is 'ty'.
'went' placed within 'ty' is 'TWENTY'.
(Other definitions for twenty that I've seen before include "No longer a teenager" , "previous clue?" , "with two crosses" , "Hundredweights to a ton" , "John B Keane wrote about many young men of this age" .)