Only one of them is likely to keep mum (4)
I believe the answer is:
dads
'them is likely to keep mum' is the definition.
I can't tell whether this defines the answer.
'only one' is the wordplay.
I cannot really see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'dds' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dads that I've seen before include "1 across 10" , "Family men" , "Fathers, in short" , "Fathers, informally" .)