Mother with one baby at least (7)
I believe the answer is:
minimum
'least' is the definition.
(similar in meaning)
'mother with one baby' is the wordplay.
'mother' becomes 'mum' (mum is a kind of mother).
'with' means one lot of letters go next to another.
'one baby' becomes 'mini' (I've seen this before).
'mum' put after 'mini' is 'MINIMUM'.
'at' is the link.
(Other definitions for minimum that I've seen before include "Smallest possible quantity" , "Lowest value" , "Lower limit" , "Lowest point" , "Nadir" .)