Take a chance fish will surface in flood (9)
I believe the answer is:
'take a chance' is the definition.
'fish will surface in flood' is the wordplay.
'fish' becomes 'luce' (luce is a type of fish).
'will surface' says the letters should be written in reverse (in a down clue, letters head to the 'surface').
'in' indicates putting letters inside.
'flood' becomes 'spate' (synonyms).
'luce' written backwards gives 'ecul'.
'ecul' placed within 'spate' is 'SPECULATE'.
(Other definitions for speculate that I've seen before include "Play the market" , "Imagine" , "Gamble - guess" , "Theorise without firm evidence" , "Buy in hope of profit, but with risk of loss" .)