Long to get leg over and begin apace (5,2)
I believe the answer is:
pitch in
'begin apace' is the definition.
'pitch in' can be an answer for 'begin' (thesaurus). I am not certain of the 'apace' bit.
'long to get leg over and' is the wordplay.
'long' becomes 'itch' (to long or itch for something is to desire it).
'to get leg' becomes 'pi' (I can't explain this - if you can you should give a lot more credence to this answer).
'over' indicates putting letters inside.
'and' becomes 'n' (common abbreviation for 'and').
'itch' put inside 'pi' is 'pitchi'.
'pitchi'+'n'='PITCH-IN'
Can you help me to learn more?
(Other definitions for pitch in that I've seen before include "Set to work" , "Eat heartily - help with some task - contribute money" , "Get to work" , "Lend a hand" , "play one's part" .)