Moderately good penning line, showing skill (5)
I believe the answer is:
flair
'skill' is the definition.
(thesaurus)
'moderately good penning line' is the wordplay.
'moderately good' becomes 'fair' (I've seen this before**).
'penning' indicates putting letters inside.
'line' becomes 'l'.
'fair' enclosing 'l' is 'FLAIR'.
'showing' acts as a link.
(Other definitions for flair that I've seen before include "Faculty" , "Style, panache" , "Stylish talent" , "Natural ability or elegance" , "Taste" .)