Poor reader initially held up - 'It's boring' ... (4)
I believe the answer is:
'boring' is the definition.
'poor reader initially held up' is the wordplay.
'poor' becomes 'bad' ('bad' can be similar in meaning to 'poor'**).
'initially' says to take the initial letters.
'held' is an insertion indicator.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
The initial letter of 'reader' is 'r'.
'bad' placed around 'r' is 'bard'.
'bard' reversed gives 'DRAB'.
'it's' is the link.
(Other definitions for drab that I've seen before include "Grey or light brown cloth" , "Drearily dull; slattern" , "Lacking brightness or colour" , "kind of tart" , "Dull - dreary" .)