Seen in church when spring is over before Easter starts (4)
I believe the answer is:
apse
'seen in church' is the definition.
(I've seen this before)
'spring is over before easter starts' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'is over' says the letters should be written in reverse.
'before' means one lot of letters go next to another.
'starts' suggests taking the first letters.
The initial letter of 'easter' is 'e'.
'spa' back-to-front is 'aps'.
'aps'+'e'='APSE'
'when' is the link.
(Other definitions for apse that I've seen before include "Offshoot from the choir, nave or aisle in a church" , "Large recess in church" , "Domed section of a church" , "Peas (anag) -- part of a church" , "Primate holds saint to be a" .)