Capable runner, fellow in period of calm almost (7)
I believe the answer is:
skilful
'capable' is the definition.
(thesaurus)
'runner fellow in period of calm almost' is the wordplay.
'runner' becomes 'ski' (ski is a kind of runner**).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'in' means one lot of letters goes inside another.
'period of calm' becomes 'lull' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'lull' with its final letter taken off is 'lul'.
'f' inserted within 'lul' is 'lful'.
'ski'+'lful'='SKILFUL'
(Other definitions for skilful that I've seen before include "Deft" , "Capable" , "exhibiting crafts" , "Very adept and proficient" , "7 [CLEVER]" .)