What started the fire at sea? (7)
I believe the answer is:
lighter
'sea?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'what started the fire' is the wordplay.
'what' becomes 'er' (I can't explain this - if you can you should believe this answer much more).
'started' is a charade indicator (letters next to each other).
'the fire' becomes 'light'.
'er' put after 'light' is 'LIGHTER'.
'at' is the link.
Can you help me to learn more?
(Other definitions for lighter that I've seen before include "carrying less cargo?" , "that leads the way for others?" , "Barge; igniter" , "- boot; - bun; - ware" , "The girl (anag.)" .)