As soon as you can locate one cell and identify the clashing letters then anagram them, you will see the theme. In my case this occurs in row three, where a 3 letter down entry (def "circuit") clashes with a 12 letter across entry (def "beyond reach"). This should lead to a cell where one letter from the down entry, when mixed with five from the across will give you a word which can be represented by a symbol. The theme should then be apparent, as should the location of another thematic cell opposite the one you've just solved in the grid. Number of clashing letters can vary.