Run round second base (she'll never strike) (5)
I believe the answer is:
nurse
'she'll never strike' is the definition.
I can't tell whether this definition defines the answer.
'run round second base' is the wordplay.
'round' is a reversal indicator.
'second' becomes 's' (common abbreviation).
'base' becomes 'e' (Euler's number - base of natural logarithms).
'run' back-to-front is 'nur'.
'nur'+'s'+'e'='NURSE'
Can you help me to learn more?
(Other definitions for nurse that I've seen before include "Care for the sick" , "Nanny" , "Suckle" , "Hospital worker" , "Cherish" .)