March in Spain relates to the sea (6)
I believe the answer is:
marine
'sea' is the definition.
(I've seen this before)
'march in spain' is the wordplay.
'march' becomes 'mar' ('Mar' can be a synonym of 'march').
'spain' becomes 'e' (abbreviation for Espana).
'mar'+'in'+'e'='MARINE'
'relates to the' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for marine that I've seen before include "Airmen (anag.)" , "Maritime - seagoing soldier" , "seaman" , "Concerning the sea" , "Concerned with the sea" .)