Travel round earth at first — rational in terms of earth science? (10)
I believe the answer is:
geological
'earth science?' is the definition.
(I've seen this before)
'travel round earth at first rational in terms' is the wordplay.
'travel' becomes 'go' (synonyms).
'round' is an insertion indicator.
'at first' indicates taking the first letters.
'rational in terms' becomes 'logical' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'earth' is 'e'.
'go' going around 'e' is 'geo'.
'geo'+'logical'='GEOLOGICAL'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for geological that I've seen before include "earth science" , "about the structure of the earth" .)