Competency rating, one linked with written material, variable (7)
I believe the answer is:
ability
'competency' is the definition.
(thesaurus)
'rating one linked with written material variable' is the wordplay.
'rating' becomes 'ab' (both can mean a sailor).
'one' becomes 'I' (roman numeral).
'linked with' means one lot of letters go next to another.
'written material' becomes 'lit' (short for literature).
'variable' becomes 'y' (I've seen this in other clues).
'ab'+'i'+'lit'+'y'='ABILITY'
(Other definitions for ability that I've seen before include "Capability, competence" , "Capacity, competence" , "Skill, talent" , "Power or skill to do" , "Genius" .)