Sing about last PM once, in short (6)
I believe the answer is:
tattle
'sing' is the definition.
(synonyms)
'about last pm once in short' is the wordplay.
'last' suggests the final letters.
'pm once' becomes 'attlee' (Clement Attlee).
'in short' means to remove the last letter.
The final letter of 'about' is 't'.
'attlee' with its final letter taken away is 'attle'.
't'+'attle'='TATTLE'
(Other definitions for tattle that I've seen before include "Chatter idly" , "Gossip, idle chatter" , "Divulge secrets" , "Tell tales" , "Inform on someone" .)