Relax, I altered some bits describing what's situated on the main line (5)
I believe the answer is:
axial
'bits describing what's situated on the main line' is the definition.
I can't judge whether this defines the answer.
'relax i altered some' is the wordplay.
'some' says the answer is hidden in the clue.
'AXIAL' is hidden within 'relax i altered'.
Can you help me to learn more?
(Other definitions for axial that I've seen before include "On a pivotal point" , "Of line about which rotation occurs" , "Relating to a line of rotation" , "pivot" , "Forming an axis" .)