When in Golf covering kilometres is demanding (6)
I believe the answer is:
asking
'is demanding' is the definition.
(I've seen this before)
'when in golf covering kilometres' is the wordplay.
'when' becomes 'as'.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'covering' is an insertion indicator.
'kilometres' becomes 'k'.
'as'+'in'+'g'='asing'
'asing' placed around 'k' is 'ASKING'.
(Other definitions for asking that I've seen before include "Making enquiries" , "Putting question or requesting" , "Inquiring" , "Appealing" , "Posing a question" .)