Birds not exactly commonplace on the golf course (6)
I believe the answer is:
eagles
'birds not' is the definition.
'eagles' can be an answer for 'birds' (I have seen 'Birds of prey ' mean 'eagles' so perhaps 'birds' could also mean 'eagles'). I am not sure about the 'not' bit.
'exactly commonplace on the golf course' is the wordplay.
'exactly commonplace' becomes 'eale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is an insertion indicator (as in clothing 'on' a person).
'the golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'course' becomes 's'.
'eale' placed around 'g' is 'eagle'.
'eagle'+'s'='EAGLES'
Can you help me to learn more?
(Other definitions for eagles that I've seen before include "Below-par" , "rock group?" , "Birds with a sea leg?" , "Such raptors" , "Large birds of prey" .)