A lake frequently high (5)
I believe the answer is:
aloft
'high' is the definition.
(I've seen this in another clue)
'a lake frequently' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'frequently' becomes 'oft' (oft can mean frequently, eg oft-quoted).
'a'+'l'+'oft'='ALOFT'
(Other definitions for aloft that I've seen before include "Animistic cult" , "To great height" , "Skyward" , "Heading for heaven" , "At a considerable height" .)