Loco observer teaches craftsman (5,7)
I believe the answer is:
train spotter
'loco observer' is the definition.
The definition suggests a singular noun which matches the answer.
'teaches craftsman' is the wordplay.
'teaches' becomes 'trains' (training is a kind of teaching).
'craftsman' becomes 'potter' (potter is a kind of craftsman).
'trains'+'potter'='TRAIN-SPOTTER'
Can you help me to learn more?
(Other definitions for train spotter that I've seen before include "Railway enthusiast" , "Anorak" , "Loco number collector" , "Transport buff" , "Anorak-clad hobbyist" .)