Trustee that's second, scores (5)
I believe the answer is:
tries
'scores' is the definition.
(scores a try in rugby)
'trustee that's second' is the wordplay.
'trustee' becomes 'tr' (abbreviation).
'that's' becomes 'ie' (abbreviation for 'id est', 'that is' in Latin).
'second' becomes 's' (common abbreviation).
'tr'+'ie'+'s'='TRIES'
(Other definitions for tries that I've seen before include "Scoring moves in rugby" , "Attempts" .)