Clue for "could become grey" (5,2)

'grey' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.

'clue for could become' is the wordplay.
'clue' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'become' indicates anagramming the letters.
'could' anagrammed gives 'cloud'.
'up' put after 'cloud' is 'CLOUD UP'.

Can you help me to learn more?