A good score, I see, can give a boost (5)
I believe the answer is:
tonic
'a boost' is the definition.
(I've seen this before)
'a good score i see' is the wordplay.
'a good score' becomes 'ton' (I can't explain this - if you can you should give a lot more credence to this answer).
'see' becomes 'c' (the word for the letter, according to Chambers).
'ton'+'i'+'c'='TONIC'
'can give' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "Invigorating medicine - keynote" , "do for musicians" , "Important note" , "__water; gin mixer" , "A pick-me-up drink, nice with gin" .)