Croatian's face when a Welshman hid (9)
I believe the answer is:
concealed
'hid' is the definition.
('conceal' can be a synonym of 'hide')
'croatian's face when a welshman' is the wordplay.
'face' suggests taking the first letters.
'when' becomes 'once' (synonyms).
'a welshman' becomes 'aled' (common Welsh name).
The first letter of 'croatians' is 'c'.
'c'+'once'+'aled'='CONCEALED'
(Other definitions for concealed that I've seen before include "Hidden" , "Not open to view" , "Not accessible to view" .)