Meant working to contain one large complaint (7)
I believe the answer is:
ailment
'complaint' is the definition.
(I know that complaint can be written as ailment)
'meant working to contain one large' is the wordplay.
'working' is an anagram indicator.
'to contain' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'large' becomes 'L' (eg in clothes sizes).
'meant' anagrammed gives 'ament'.
'i'+'l'='il'
'ament' placed around 'il' is 'AILMENT'.
(Other definitions for ailment that I've seen before include "Complaint, affliction" , "Physical disorder" , "Slight but persistent illness" , "Tim Alen has an odd indisposition." , "Minor illness" .)