A poster’s oddly seen in recesses (5)
I believe the answer is:
apses
'recesses' is the definition.
(apse is a kind of recess)
'a poster's oddly seen' is the wordplay.
'oddly seen' indicates alternate letters (letters in odd-numbered positions are used).
The alternate letters of 'posters' are 'pses'.
'a'+'pses'='APSES'
'in' acts as a link.
(Other definitions for apses that I've seen before include "Domed areas in church, usually where the altar is" , "Does 'e pass these in churches?" , "Parts of church" , "Domed areas in churches" , "Domed areas in church, usually where the altars are" .)