Strange thing swallowed by fish in a flash (9)
I believe the answer is:
lightning
'a flash' is the definition.
(I know that lightning is a type of flash)
'strange thing swallowed by fish' is the wordplay.
'strange' indicates anagramming the letters.
'swallowed by' indicates putting letters inside.
'fish' becomes 'ling' (ling is a kind of fish).
'thing' is an anagram of 'ightn'.
'ightn' put within 'ling' is 'LIGHTNING'.
'in' acts as a link.
(Other definitions for lightning that I've seen before include "Weather featuring with 9" , "Precaution against electrical storms" , "Flash in the sky" , "Discharge of electricity in the atmosphere" , "Atmospheric electricity" .)