Bird sitting on church, unknown and very small (6)
I believe the answer is:
titchy
'very small' is the definition.
(I've seen this before)
'bird sitting on church unknown' is the wordplay.
'bird' becomes 'tit' (I've seen this before**).
'sitting on' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'unknown' becomes 'y' (algebra).
'tit'+'ch'+'y'='TITCHY'
'and' acts as a link.
(Other definitions for titchy that I've seen before include "Tiny" , "Very short" , "Little" , "Very small" .)