Not as many women get into free running (5)
I believe the answer is:
fewer
'not as many' is the definition.
(similar in meaning)
'women get into free running' is the wordplay.
'women' becomes 'w' (standard abbreviation).
'get into' means one lot of letters goes inside another.
'running' is an anagram indicator.
'free' is an anagram of 'feer'.
'w' put within 'feer' is 'FEWER'.
(Other definitions for fewer that I've seen before include "smaller number of people" , "in smaller numbers" , "Not so many" , "Less in number" , "Reduced" .)