At half pace, could be merely a partial test (5)
I believe the answer is:
trial
'a partial test' is the definition.
'trial' can be an answer for 'test' (synonyms). I am not sure about the 'a partial' bit.
'at half pace could be merely' is the wordplay.
'at' means one lot of letters go next to another.
'half' suggests halving the letters.
'pace' becomes 'trot' (I've seen this before).
'could be merely' becomes 'ial' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trot' with half the letters taken is 'tr'.
'tr' next to 'ial' is 'TRIAL'.
Can you help me to learn more?
(Other definitions for trial that I've seen before include "Troublesome thing; test" , "Court proceedings" , "Dummy run" , "Ordeal, perhaps in court" , "Annoyance" .)