Sky feature, series inspiring a popular book (7)
I believe the answer is:
rainbow
'sky feature' is the definition.
(something in the sky)
'series inspiring a popular book' is the wordplay.
'series' becomes 'row' (I've seen this before).
'inspiring' indicates putting letters inside.
'popular' becomes 'in' ('in' can mean trendy).
'book' becomes 'b' (abbreviation for book).
'a'+'in'+'b'='ainb'
'row' going around 'ainb' is 'RAINBOW'.
(Other definitions for rainbow that I've seen before include "'I can sing a - - ' (song)" , "illusory hope?" , "Coloured arc in the sky" , "heavenly sight" , "in different colours" .)