Fixed, as nails? Quickly too! (4,3,4)
I believe the answer is:
hard and fast
'fixed' is the definition.
(I've seen this before)
'nails? quickly too' is the wordplay.
I cannot really see how this works, but
'quickly' could be 'fast' ('fast' can be a synonym of 'quickly') and 'fast' is located in the answer.
'too' could be 'and' and 'and' is found in the answer.
The remaining letters 'hard' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for hard and fast that I've seen before include "(Of rules) invariable" , "like a cannonball" , "Rigidly applied" , "Not moving" , "(Of rules) unalterable" .)