Old lady carries you across the Channel in fact (5)
I believe the answer is:
datum
'channel in fact' is the definition.
'datum' can be an answer for 'fact' (singular form of 'data'). I'm unsure of the 'channel in' bit.
'old lady carries you across the' is the wordplay.
'old lady' becomes 'dam' (I can't explain this - if you can you should give a lot more credence to this answer).
'carries' is an insertion indicator (inserted letters carried inside).
'you' becomes 'u' (commonly used in online communication).
'across' means one lot of letters goes inside another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'dam' enclosing 'u' is 'daum'.
'daum' placed around 't' is 'DATUM'.
Can you help me to learn more?
(Other definitions for datum that I've seen before include "Given piece of information" , "Premiss, thing taken for granted" , "Single informative fact" , "bit of info" , "Fixed starting point for measurements" .)