Perhaps I'd come about something commonplace (8)
I believe the answer is:
mediocre
'commonplace' is the definition.
(I've seen this before)
'perhaps i'd come about something' is the wordplay.
'perhaps' is an anagram indicator.
'about' shows that the letters should be reversed in order.
'something' becomes 'er' (I've seen this in other clues).
'er' back-to-front is 're'.
'id'+'come'='idcome'
'idcome' with letters rearranged gives 'medioc'.
'medioc'+'re'='MEDIOCRE'
(Other definitions for mediocre that I've seen before include "Pedestrian, second-rate" , "Rather inferior" , "Of middling quality" , "Second-rate, undistinguished" , "Middling or average" .)