Footman could have no key to speak of (6)
I believe the answer is:
lackey
'footman' is the definition.
(I've seen this before)
'key to speak of' is the wordplay.
'to' means one lot of letters go next to another (I've seen this in other clues).
'speak of' becomes 'lac' (I can't explain this - if you can you should give a lot more credence to this answer).
'key' put after 'lac' is 'LACKEY'.
'could have no' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lackey that I've seen before include "assistant" , "An archaic servant or underling" , "Liveried servant" , "An archaic menial or servant" , "An old servant or slavish person" .)