Returning sailor gets sibling foundation (5)
I believe the answer is:
basis
'foundation' is the definition.
(I know that basis is a type of foundation)
'returning sailor gets sibling' is the wordplay.
'returning' shows that the letters should be reversed in order.
'sailor' becomes 'ab' (abbreviation for able seaman).
'gets' says to put letters next to each other.
'sibling' becomes 'sis' (short for sister).
'ab' written backwards gives 'ba'.
'ba'+'sis'='BASIS'
(Other definitions for basis that I've seen before include "Principle on which a theory is founded" , "Foundation of a theory or process" , "Underlying support for a theory" , "Foundation, fundament" , "Fundamental part" .)