When in Golf covering kilometres is demanding (6)
'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'.
'asing' placed around 'k' is 'ASKING'.
(Other definitions for asking that I've seen before include "Posing a question", "For the ___; freely available", "Inquiry", "Appealing", "Making request".)