Unusual reaction as drugs periodically found in lane (7)
I believe the answer is:
allergy
'unusual reaction' is the definition.
'allergy' can be an answer for 'reaction' (I've seen this before). I'm not certain of the 'unusual' bit.
'drugs periodically found in lane' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'found in' indicates putting letters inside.
'lane' becomes 'alley' (alley is a kind of lane).
The alternating letters of 'drugs' are 'rg'.
'rg' going into 'alley' is 'ALLERGY'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for allergy that I've seen before include "Adverse reaction (med.)" , "Unfavourable reaction to pollen, certain foods etc" , "Hypersensitive reaction of the body to a foreign substance" , "Largely in an unfavourable reaction, maybe to pollen" , "Pathological aversion" .)