When I had a key in reserve (5)
I believe the answer is:
aside
'in reserve' is the definition.
(synonyms)
'when i had a key' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'i had' becomes 'i'd' (contraction).
'a key' becomes 'E' (a musical key).
'as'+'id'+'e'='ASIDE'
(Other definitions for aside that I've seen before include "private" , "Digression" , "Privately, apart" , "Actor's line directed solely to audience" , "Line spoken to the audience of a play" .)