Sailor with old knight in desert (7)
I believe the answer is:
abandon
'desert' is the definition.
(synonyms)
'sailor with old knight' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'with' becomes 'and'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'knight' becomes 'N' (chess abbreviation).
'ab'+'and'+'o'+'n'='ABANDON'
'in' is the link.
(Other definitions for abandon that I've seen before include "Walk out on" , "Forsake with recklessness" , "Desert; intemperance" , "Wantonness: desert" , "Stop" .)