Poet's current service inspires old long-distance caller (7)
I believe the answer is:
alphorn
'long-distance caller' is the definition.
The definition suggests a singular noun which matches the answer.
'poet's current service inspires old' is the wordplay.
'poet's current' becomes 'alph' (I can't explain this - if you can you should believe this answer much more).
'service' becomes 'RN' (abbreviation for Royal Navy).
'inspires' means one lot of letters goes inside another (inspire means to breathe in).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'alph'+'rn'='alphrn'
'alphrn' placed around 'o' is 'ALPHORN'.
Can you help me to learn more?
(Other definitions for alphorn that I've seen before include "Mountain instrument" , "Instrument of mountainous regions" .)