Vietnam allegedly holding back source of wool (5)
I believe the answer is:
llama
'source of wool' is the definition.
(I've seen this before)
'vietnam allegedly holding back' is the wordplay.
'holding' indicates a hidden word.
'back' says the letters should be written in reverse.
'amall' can be found hidden inside 'vietnam allegedly'.
'amall' backwards is 'LLAMA'.
(Other definitions for llama that I've seen before include "South American relative of the camel" , "Andean 'camel'" , "Pack carrier" , "Even-toed camel relative" , "One bearing burden" .)