The unclued lights (one of three words and two of two words), individually or as a pair, are of a kind.
 

Across

 
5    Bit of tiffin is hardly perfect (6)
12    Fail to see active service (5)
13    Waistcoat cloth that’s not in ablutions (8)
16    Beer mug in set cracked (5)
17    Mourns for old sheriffs (7)
18    Additionally suggest Tory memo (7)
20    Civic dignitary, a lord, with topless Berliner, say (8)
26    Extra portion of lamb? Yes! (3)
27    Tailless bird – dead duck in the Loire (6)
29    Aussie expert involved with 8 for 1A (3)
30   ...