The unclued lights can be arranged into a chain of anagrams of 3 to 11 letters in length, with the addition of one letter step by step.
 

Across

7 Towboat capsized in channel (3)
11 Contain grease once (6)
13 Against work — was perplexed (7)
15 Bender on river (5)
16 Two flowers of sorts in colleges (5)
17 Prudently state diocese (6)
20 Hymn to alluring female without power (6)
21 Associated feature that is in the can (5, hyphened)
27 Artificial swap of Mafia member (7, hyphened)
29 Warm-up races! (5)
30 One hug could become sufficient (6)
32 Great desire of ogre, edified (5)
34 Herbie’s mallet for game (6)
36...