The older Spaniard is out of line! (6)
I believe the answer is:
senior
'the older' is the definition.
(I've seen this before)
'spaniard is out of line' is the wordplay.
'spaniard' becomes 'senor' ('mister' in Spanish).
'is out' means one lot of letters goes inside another.
'of line' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'senor' placed around 'i' is 'SENIOR'.
Can you help me to learn more?
(Other definitions for senior that I've seen before include "Denoting older people" , "Elder person - higher in rank" , "More advanced in years or authority" , "Higher in rank or age" , "Older or higher in standing" .)