Understand only half of what's said (3)
I believe the answer is:
ken
'understand' is the definition.
(I've seen this in another clue)
'only half of what's said' is the wordplay.
'only half of' means to take half the letters.
'what's said' becomes 'spoken' (I've seen this before).
'spoken' halved is 'KEN'.
(Other definitions for ken that I've seen before include "Understanding - boy's name" , "Know; man's name" , "One's range of knowledge" , "Identify; -- Livingstone" , "Notice" .)