A boy cracking O-level casually is worthy of affection (8)
I believe the answer is:
loveable
'worthy of affection' is the definition.
The definition suggests an adjective which matches the answer.
'a boy cracking o-level casually' is the wordplay.
'a boy' becomes 'ab' ().
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'casually' indicates an anagram (I've seen this in other clues).
'o'+'level'='olevel'
'olevel' anagrammed gives 'lovele'.
'ab' put into 'lovele' is 'LOVEABLE'.
'is' acts as a link.
Can you help me to learn more?
(Another definition for loveable that I've seen is " Attractive, endearing".)