Joined the regiment in time (5)
I believe the answer is:
metre
'time' is the definition.
(the rhythmic pattern of a piece of music or poetry)
'joined the regiment' is the wordplay.
'joined' becomes 'met' (meeting is a kind of joining).
'the regiment' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'met'+'re'='METRE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "100cm" , "Distance" , "39.37 inches" , "Basic unit of length" , "Rhythmic arrangement of syllables in verse" .)