Nine unclued lights each contain a different 20 from the same source. The title suggests a further such 20. Elsewhere, ignore an accent.
 

Across

 
1    First off clochards rummaged Mac’s valises (8)
8    Eastern sailing boat or ship in poem declaimed (4)
12    Carnal caper giving sensuous diversion (10, two words)
13    Buffaloes perhaps or bulls heartless scholar trapped (6)
14    Abbot and priest get on well (7)
15    Trumpets of tribes besieging seaport (8)
18    Moulds American gum endures (9)
19    Jellyfish start to invade Ireland, Britain and Japan first (9)
21    Mere guesswork about mushrooms (4)
29    Nanny endlessly hugs large tree (4)
30    Myself and Fred stripped bare in pool...