Point frequently and observe this weakness (4,4)
I believe the answer is:
soft spot
'this weakness' is the definition.
(thesaurus)
'point frequently and observe' is the wordplay.
'point' becomes 's' (S is an example).
'frequently' becomes 'oft' (I've seen this before).
'and' means one lot of letters go next to another.
'observe' becomes 'spot' (I've seen this before**).
's'+'oft'+'spot'='SOFT SPOT'
(Other definitions for soft spot that I've seen before include "Affectionate weakness for (someone/something)" , "Sentimental fondness" , "showing fondness" , "Emotional susceptibility" , "Sentimental affection" .)