Carefully observe a keen tot exercising (4,4)
I believe the answer is:
take note
'carefully observe' is the definition.
'take note' can be an answer for 'observe' (I've seen this in another clue). I am not certain of the 'carefully' bit.
'a keen tot exercising' is the wordplay.
'exercising' indicates an anagram (letters exercise or move around).
'a'+'keen'+'tot'='akeentot'
'akeentot' with letters rearranged gives 'TAKE NOTE'.
Can you help me to learn more?
(Other definitions for take note that I've seen before include "what the stenographer does" , "mark?" , "Observe closely" , "Observe and absorb - jot it down" , "Pay attention" .)