Racehorse runs into lake, uncontrollably (5)
I believe the answer is:
arkle
'racehorse' is the definition.
(I've seen this before)
'runs into lake uncontrollably' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' is an insertion indicator.
'uncontrollably' is an anagram indicator (I've seen this in other clues).
'lake' with letters rearranged gives 'akle'.
'r' inserted within 'akle' is 'ARKLE'.
(Other definitions for arkle that I've seen before include "Irish winner of the Cheltenham Gold Cup in 1964, 1965 and 1966" , "Famous steeplechaser" , "Classic racehorse" , "Famous racehorse" .)