Straying, sailor helping to keep direction right (10)
I believe the answer is:
aberration
'straying' is the definition.
(I've seen this before)
'sailor helping to keep direction right' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'helping' becomes 'ration' (I've seen this before).
'to keep' indicates putting letters inside.
'direction' becomes 'e' (E is an example).
'right' becomes 'r' (common abbreviation).
'ab'+'ration'='abration'
'e'+'r'='er'
'abration' enclosing 'er' is 'ABERRATION'.
(Other definitions for aberration that I've seen before include "Deviation from normality" , "Anomaly -- a Breton air (anag)" , "instability" , "Mental lapse" , "Slip" .)