Old lady can start to say prayer (6)
I believe the answer is:
matins
'prayer' is the definition.
(thesaurus)
'old lady can start to say' is the wordplay.
'old lady' becomes 'ma'.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'start to' suggests taking the first letters.
The first letter of 'say' is 's'.
'ma'+'tin'+'s'='MATINS'
(Other definitions for matins that I've seen before include "Isn't ma given to morning prayers?" , "C of E's daily morning service" , "Morning prayers or bird-songs" , "Church service" , "Anglican morning service" .)