Entering water, I function for a very short time (10)

'very short time' is the definition.
(a trillionth of a second)

'entering water i function for a' is the wordplay.
I cannot quite see how this works, but
'i' is found in the answer.
'function' could be 'cosec' (I've seen this before) and 'cosec' is found within the answer.
'water' could be 'pond' (pond is a kind of water) and 'pond' is found in the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).