Cautious of administering a drug as a boy (7)
I believe the answer is:
Here is my best explanation:
'cautious' is the definition.
('guarded' can be similar in meaning to 'cautious')
'administering a drug as a boy' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
an anagram of 'drug' is 'gurd' which is within the remaining letters.
'boy' could be 'ed' and 'ed' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'of' acts as a link.
(Other definitions for guarded that I've seen before include "Protected from a drudge", "Kept watch over", "Cagey or protected", "with escort", "Reticent", "Cautious; watched over", "wary", "offered protection", "Wary" and "Cautious - or protected".)