Spot Mike on rescue boat (4)
I believe the answer is:
mark
'spot' is the definition.
(I know that spot is a more specific form of mark)
'mike on rescue boat' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'rescue boat' becomes 'ark' (rescue boat during the biblical flood).
'm'+'ark'='MARK'
(Other definitions for mark that I've seen before include "Trace" , "The second Gospel" , "made by those who hit the big time" , "Matthew, . . . ., Luke and John" , "Blemish" .)