Said in the letter the bird, a tit, had flown in (7)
I believe the answer is:
written
'said in the letter' is the definition.
I can't tell whether this definition defines the answer.
'bird a tit had flown in' is the wordplay.
I cannot really understand how this works, but
an anagram of 'tit' is 'itt' which is within the answer.
'bird' could be 'wren' (wren is a kind of bird) and 'wren' is located in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for written that I've seen before include "Expressed on paper" , "Jotted down" , "Committed to paper" , "taken down" , "Transcribed" .)