Kiwi's heading beyond valley — it can't take flight! (5)
I believe the answer is:
dalek
'it can't take flight' is the definition.
(in Doctor Who, Daleks can't go up stairs)
'kiwi's heading beyond valley' is the wordplay.
'heading' indicates taking the first letters (head could mean the top letter).
'beyond' is a charade indicator (letters next to each other).
'valley' becomes 'dale' (dale is a kind of valley).
The first letter of 'kiwis' is 'k'.
'k' put after 'dale' is 'DALEK'.
(Other definitions for dalek that I've seen before include "Croaky-voiced exterminator" , "mechanical creature" , "Talking robot" , "Robot from Doctor Who" , "Foe of Dr Who" .)