Solve clue involving firm yet defective cell
I believe the answer is:
leucocyte
'cell' is the definition.
(leucocyte is a kind of cell)
'solve clue involving firm yet defective' is the wordplay.
'solve' indicates an anagram (I've seen 'solved' mean this (the anagram must be solved)).
'involving' means one lot of letters goes inside another.
'firm' becomes 'co' (a firm is a company).
'defective' indicates anagramming the letters.
'clue' with letters rearranged gives 'leuc'.
'leuc' going around 'co' is 'leucoc'.
'yet' is an anagram of 'yte'.
'leucoc'+'yte'='LEUCOCYTE'
(Another definition for leucocyte that I've seen is " cell".)