Clue: Leaving to collect small women's group, not even mums, to take a dip (2,8)
Answer: GO SWIMMING
Explanation: Definition: to take a dip
 GOING(leaving/departing from) **containing**(to collect) [ S(abbrev. for "small") + WI(abbrev. for the Women's Institute, a group/organisation of women) + **2nd and 4th letters deleted from**(not even) "mums".