Capricorn is one theme absorbing astrologer ultimately (6)
I believe the answer is:
tropic
'capricorn is one' is the definition.
(I've seen this before)
'theme absorbing astrologer ultimately' is the wordplay.
'theme' becomes 'topic' ('topic' can be a synonym of 'theme').
'absorbing' indicates putting letters inside.
'astrologer ultimately' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'topic' placed around 'r' is 'TROPIC'.
Can you help me to learn more?
(Other definitions for tropic that I've seen before include "Imaginary circle on globe" , "Either that of Capricorn or Cancer" , "Interzonal line" , "Line defining hot zone" , "Cancer/Capricorn line" .)