Comparatively adept sailor with large margins of error (5)
I believe the answer is:
abler
'comparatively adept' is the definition.
(more able)
'sailor with large margins of error' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'with' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'margins of' says to hollow out the word (remove centre letters) (only the letters on the margins of the word).
'error' with its middle removed is 'er'.
'ab'+'l'+'er'='ABLER'
(Other definitions for abler that I've seen before include "stronger" , "Increasingly smart" , "More competent, perhaps like seaman" , "More skilful" , "More skilled" .)