Social exclusion also takes in a mariner (5)
I believe the answer is:
taboo
'social exclusion' is the definition.
'taboo' can be an answer for 'exclusion' (thesaurus). I am not sure about the 'social' bit.
'also takes in a mariner' is the wordplay.
'also' becomes 'too' (synonyms).
'takes in' is an insertion indicator.
'a mariner' becomes 'AB' (short for able seaman).
'too' placed around 'ab' is 'TABOO'.
Can you help me to learn more?
(Other definitions for taboo that I've seen before include "Socially forbidden" , "Inhibition based on social custom" , "Ritually forbidden" , "ruled out" , "By religion or custom, not to be done or touched" .)