Most new mums do it, tardily pocketing bill (7)
I believe the answer is:
lactate
'most new mums do it' is the definition.
(lactation is breastfeeding)
'tardily pocketing bill' is the wordplay.
'tardily' becomes 'late' (synonyms).
'pocketing' is an insertion indicator.
'bill' becomes 'act' (I've seen this in other clues).
'late' placed around 'act' is 'LACTATE'.
(Other definitions for lactate that I've seen before include "(Of female mammal) secrete milk" , "Produce milk" , "Exude milk" .)