An increasing problem ties boy in knots (7)
I believe the answer is:
obesity
'an increasing problem' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'ties boy in knots' is the wordplay.
'in knots' is an anagram indicator (I've seen this in other clues).
'ties'+'boy'='tiesboy'
'tiesboy' with letters rearranged gives 'OBESITY'.
Can you help me to learn more?
(Other definitions for obesity that I've seen before include "Fatness to great excess" , "Bulk" , "Abnormal fatness" , "Unhealthy excess of fatness" , "Condition of excessive fatness" .)