1a. An advantage or positive quality (4,4) P?U? C?G?
17d. Leadership or advice on a problem or difficulty (8)
?U?D????
24a. Noun phrase functioning as the agent of a he verb (6)
A?T???
26a. A boisterous girl, once (6). H?R???
In Chambers light is defined as "in a crossword the word (or sometimes the individual letter in the word) that is the answer to the clue.", so looks OK.